Renal Profile and Outcome of Patients With Post-Obstructive Diuresis at the UST Hospital: A Retrospective Study

Introduction

Post-obstructive diuresis (POD) is a condition characterized by excessive urine output following the relief of urinary tract obstruction, typically after decompression of an obstructed bladder or ureter. The diagnostic criteria for POD include urine production exceeding 200 mL per hour for two consecutive hours or greater than 3 L within 24 hours.[1] It involves polyuria, where large amounts of water and salt are excreted after urinary obstruction relief. The incidence of POD is uncertain, with estimates ranging from 0.5% to 52%.[2]

POD typically occurs after alleviating bladder outlet obstruction, bilateral ureteral obstruction, or unilateral ureteral obstruction in a solitary kidney. Diuresis serves as a normal physiological response to eliminate excess volume and solutes accumulated due to obstruction. In most patients, diuresis resolves once the kidneys restore balance.[3] Polyuria is more common after bilateral ureteral or subvesical obstruction, while unilateral obstruction rarely causes POD. Thiel, et.al. reported an extreme case with 58,000 mL per day of diuresis (204 L in 10 days) following left ureteral obstruction relief.[3] The pathophysiology of POD is not fully understood, but it is believed to involve multiple factors. Treatment focuses on careful monitoring and fluid-electrolyte replacement.[3] 

Despite its clinical significance, the risk factors for POD remain poorly defined. Factors like lower urinary tract symptoms, diabetes, prior urethral catheterizations, prostatic hyperplasia, fecal impaction and anticholinergic medication use have been identified as potential risk factors.[2] However, study results are inconsistent.[4,5] Several mechanisms have been proposed for POD's pathophysiology, including vascular washout, down-regulation of sodium transporters in the loop of Henle, GFR reduction and impaired antidiuretic hormone response. POD likely results from a combination of these factors.[2] A literature review by Shah, et.al. highlighted tubular dysfunction and recovery as key elements in POD's transient nature. Urinary output and electrolyte excretion peak within 24 hours and typically stabilize by day 14.[6] 

Patients with POD are at risk for severe dehydration, electrolyte imbalances, hypovolemic shock and, in extreme cases, death.[7] Predicting which patients will develop POD after urinary tract decompression remains challenging. Hamdi, et.al. identified high serum creatinine levels, elevated sodium bicarbonate and urinary retention as independent risk factors for developing POD.[8] However, few markers predict whether POD will progress to a pathologic state. Some studies have linked renal insufficiency, heart failure, volume overload, dizziness and central nervous system (CNS) depression as risk factors for severe POD.[9] 

Management of POD generally involves fluid replacement, tailored based on clinical parameters, diuresis and monitoring. High-risk patients should be closely observed, and nephrologist involvement may be necessary for prolonged diuresis.[6] While POD is common in urology, vigilant management is required. Understanding its pathophysiology, risks and management strategies is crucial for clinicians, particularly internists. No formal guidelines exist for diagnosing or managing POD, though further research could establish a clearer framework for clinical practice.

Literature on POD profiles and outcomes is limited. Bermeo, et.al. examined 88 patients with ureteropelvic junction obstruction who underwent open pyeloplasty between 2006 and 2016. About 30% developed POD, with younger age and tubular acidosis correlated with its development.[7] Despite analyzing serum electrolytes, they did not correlate with POD.[10] 

Frohlich, et.al. assessed POD incidence in cats after urethral obstruction relief, finding 88% developed POD within 48 hours, with intravenous fluids (IVF) likely contributing to high polyuria.[11] Francis, et.al. explored the correlation between venous pH and POD, finding that POD was more likely in samples with venous pH <7.35.[12] Wilson, et.al. studied natriuretic factors in rats, showing that high blood and urine urea levels contributed to pronounced diuresis, explaining differences between bilateral and unilateral ureteral relief.[13] 

Quinlan, et.al. (2010) compared gender-based renal responses in rats following unilateral obstruction and nephrectomy. They found increased renal injury and reduced glomerular filtration rate (GFR) in male rats but no significant differences in other parameters, although both sexes exhibited increased aquaporin-2 expression.[14] Gillenwater’s 1975 study found no cases of POD after unilateral hydronephrosis relief, despite some obstructed kidneys showing impaired GFR and sodium reabsorption.[15] 

A 2012 study by Hamdi, et.al. found that POD occurred more frequently in patients with AKI, even without chronic renal failure. Risk factors for POD in chronic renal failure patients included lower hemoglobin, serum bicarbonate levels, delayed relief and the absence of POD.[8] 

This study aims to provide insights for patients with obstructive nephropathy or uropathy preparing for decompressive surgery, improving patient preparation, outcome prediction and complication management. It will also assist general practitioners and physicians in preoperative clearance and risk stratification.

The primary objective is to evaluate the renal profile, risk factors and outcomes of POD in patients treated at the University of Santo Tomas Hospital between January 2017 and December 2018. Specifically, the study aims to identify blood and urine parameters influencing or predicting POD, assess fluid therapy's role, determine POD incidence and explore correlations with serum creatinine levels, electrolyte imbalances and creatinine clearance. This analysis will enhance the understanding and management of POD in this patient population.

Methods

This study retrospectively analyzed POD patients admitted to the University of Santo Tomas Hospital from January 2017 to December 2018. Patients who met the criteria for POD were included.

Study Population

Demographic, etiological and clinical data for hospitalized patients diagnosed with obstructive uropathy (bilateral or unilateral) at the UST Hospital were collected. Among these, cases diagnosed with POD were selected. The data came from inpatient charts at the UST Hospital Records Section.

Patients aged >18 diagnosed with POD were included. POD was defined as urine production exceeding 200 mL per hour for two consecutive hours or greater than 3 L of urine in 24 hours, based on Leslie's 2018 study. Patients had to be admitted and observed for more than 24 hours at the UST Hospital.

Patients with these findings before the operation or discharged in less than 24 hours were excluded, along with those with unilateral obstruction, except for those with a solitary kidney.

Study Procedure

The obstructive uropathy database from January 2017 to December 2018 was reviewed. Researchers identified patients who underwent procedures to relieve the obstruction. A chart review identified patients diagnosed with POD.

Necessary data were extracted and ethical considerations will be discussed in the Ethical Consideration section. Data were analyzed to identify risk factors and outcomes, including length of hospital stay, days to resolve POD and electrolyte imbalances. Included risk factors were age, gender, sodium, creatinine and Blood urea nitrogen (BUN) levels before surgery, ultrasound findings, urinalysis results (pH, specific gravity, urine chemistry and microscopy), IVF volume and use of antibiotics before surgery.

Data were encoded using a data-gathering instrument developed for this study and Microsoft Excel.

Sampling Design and Sample Size

The study identified all patients with POD from January 2017 to December 2018 in the UST Hospital, including all of them with approval from the Institutional Review Board (IRB) to review their charts and data.

Statistical Analysis

Clinical, biological and imaging characteristics on admission and post-obstruction release were analyzed using univariate and multivariate analysis to identify predictors of POD occurrence. Data were presented as percentages. A p-value <0.05 was considered statistically significant.

Descriptive statistics summarized the patients' demographic and clinical characteristics. Frequency and proportion were used for categorical variables as well as median and interquartile range used for non-normally distributed continuous variables. For normally distributed variables, the mean and standard deviation (SD) were used. Independent Sample T-test, Mann-Whitney U test and Fisher’s Exact/Chi-square tests were used to determine the mean, rank and frequency differences between patients with and without POD. Wilcoxon signed-rank test assessed changes from preoperative to postoperative serum creatinine, Na, K, eGFR and iCA. Odds ratios and 95% confidence intervals from binary logistic regression were calculated to determine factors associated with POD. All tests were two-tailed. Shapiro-Wilk was used for normality testing and missing variables were not estimated. Null hypotheses were rejected at a 0.05 significance level. Data analysis was performed using STATA 13.1.

Data Collection/Gathering Tools

A master table was made to be used for encoding data. An individual patient sheet/instrument with a checklist of risk factors and outcomes was also created for each chart. Terms found on the data gathering instrument were properly defined (including operational definitions for the coding process).

Figure 1. Schematic Diagram of the Research Paper

ETHICAL CONSIDERATION

The study received approval from the Research Ethics Committee (REC) of UST Hospital. The medical director was informed, and the study followed the 2017 National Ethical Guidelines for Health and Health-Related Research (NEGHHRR).

INFORMED CONSENT

Since this study was a chart review, patient informed consent was not required for using data from records. According to the NEGHHRR, informed consent may be waived for specific research contexts such as archival research. Ethical principles outlined by Sakar in "Conducting Record Review Studies in Clinical Practice" were followed. This included obtaining only necessary information, ensuring confidentiality, removing identifying data, and seeking approval from the IRB.

The study requested USTH-REC to waive the informed consent process based on these principles.

COMPLIANCE WITH THE DATA PRIVACY ACT

The study ensured confidentiality. Only authorized researchers could access the data, and subjects were assigned alpha-numeric codes for anonymity. Coded data were sent to the statistician for analysis. The study adhered to the Data Privacy Act of 2012 and its Implementing Rules and Regulations of 2016.

DATA MANAGEMENT

Data were used solely for research purposes and stored on a secure hard drive accessible only to researchers and statisticians. Data were processed fairly and retained as long as necessary for the research. Records were to be retained for three years, with proper disposal afterward: hard copies were shredded, and soft copies were deleted. Flash drives would be reformatted.

BENEFITS

No direct benefits were given to patients. However, the study’s results could advance the understanding and management of POD in similar patients.

RISKS

Patients faced no direct risks except for potential breaches in confidentiality. However, the risk was minimized by following the Data Privacy Act of 2012.

CONFLICT OF INTEREST

There was no conflict of interest

Results

After retrieving records of patients previously admitted at the University of Santo Tomas Hospital in 2018 using the search terms mentioned above, a total of 182 charts were retrospectively reviewed. Of these, only 106 had any of the following: bilateral ureteral obstruction, bladder outlet/ urethral obstruction (including prostate pathologies), or unilateral obstruction on a patient with a solitary kidney. Among these patients, the incidence of post-obstructive diuresis is 28.3% (  n=30  ).

CLINICAL PROFILE OF POST-OBSTRUCTIVE DIURESIS (POD)

The mean age of patients who had post-obstructive diuresis is 58 years old, ranging from 44 to 72 years. Those patients who did not have POD have an almost equal mean age of 58 years. There was no significant difference between these two groups with regard to age (p=0.967). The majority of patients diagnosed with POD are male, comprising 63.33% (  n= 19  ) of the group. Female gender, on the other hand, predominates patients who did not develop POD at 51.32%.

Co-morbid illnesses among patients who had POD were likewise identified. The three most common illnesses were smoking, acute kidney injury, and chronic kidney disease (n=14,11,10, respectively). Less commonly, other co-morbidities include hypertension, diabetes mellitus, and urinary tract infection. Body mass index (BMI) was also calculated among POD patients. In the POD group, the majority belongs to the overweight classification (56.67%,  n=17  ) as compared to those in the non-POD group, in which most have a normal BMI (65.33%,  n=49  ) [p=<0.001].

Table 1. Demographic Profile of Patients Diagnosed with Post-Obstructive Diuresis (POD)

^a includes nephrolithiases (such as staghorn calculi), urolithiases in any part of the urinary tract.

^b genitourinary tuberculosis, neurologic pathologies (such as neurogenic bladder, DM cystopathy), blood clots and pelvic inflammatory disease among females

^c pertains to the number of days from the time of obstructive symptoms until any interventions to remove obstruction.

^d pertains to the number of days with post-obstructive diuresis until resolution.

^e any smoking history (not specific with pack years)

^f defined by the KDIGO guidelines of Acute Kidney Injury and Chronic Kidney Injury

^g type and duration of operation/surgery/intervention done to relieve the obstruction

^h based on the Asia Pacific Classification of Body Mass Index (BMI).

Acronyms: PCN—Percutaneous Nephrolithotomy, DJ—Double J, TURP—Transurethral Resection of Prostate, BMI—Body Mass Index

Diagnoses identified from the data review include lithiasis, benign prostatic hypertrophy/enlargement (BPH), prostate cancer, urinary tract infection causing acute urinary retention, ureteral/urethral stricture, obstruction from lymph nodes, and cancer infiltrations. The most common condition leading to post-obstructive diuresis (POD) after decompression is lithiasis in any part of the urinary tract (e.g., bilateral staghorn calculi, bilateral ureteral lithiasis, unilateral lithiasis in a solitary kidney, and less commonly, bladder lithiasis). Thirty-three percent (33%) of patients developed POD after an intervention to relieve obstruction. The second most common diagnosis for POD patients is prostate cancer (26.67%, n=8). Obstructions in the ureters or urethra did not lead to POD after intervention, nor did lymph node obstruction. Most obstructions from lithiasis are nephrolithiasis, primarily bilateral staghorn calculi.

Ten percent (10%) of POD patients had obstruction secondary to BPH (n=3), and 3.33% (n=1) had acute urinary retention due to a urinary tract infection that led to POD after Foley catheter insertion.

Other less common diagnoses associated with POD include genitourinary tuberculosis (GUTB), neurologic pathologies (e.g., neurogenic bladder, DM cystopathy), retained blood clots, and unspecified pelvic inflammatory disease in females. Decompressive interventions, such as indwelling Foley catheters, bilateral DJ stent insertion, or other invasive procedures, were used to relieve obstruction. The most common intervention was the insertion of an indwelling Foley catheter with hourly urine output monitoring (46.67%, n=25). This was followed by bilateral or unilateral DJ stents. There was a significant difference in DJ stent insertion rates between POD and non-POD groups (p=0.021), with all unilateral kidney DJ stent insertions leading to POD (10%, n=3).

The duration of obstruction was analyzed, showing that POD patients had significantly longer obstruction durations than those without POD (p=0.049). The average duration of obstruction before intervention in the POD group was 14 days (range: 4–30 days). Most patients who did not develop POD had less than seven days of obstruction.

RENAL PROFILE OF POST-OBSTRUCTIVE DIURESIS (POD)

Serum creatinine, sodium, potassium, ionized calcium, and urea nitrogen were analyzed and correlated among these patients. Arterial pH, arterial bicarbonate level and urine pH, specific gravity, and albumin were also recorded.

Table 2. Renal Profile of Patients Diagnosed with Post-Obstructive Diuresis (POD)

^a based on the Acute Kidney Injury criteria of the KDIGO

^b serum levels upon admission and prior to decompression

^c creatinine clearance based on CKD-EPI (Expressed in mL/min/1.73m²)

In general, most patients with obstructive uropathy did not have baseline acute kidney injury (AKI) (59.43%, n=63). However, a majority of those who developed post-obstructive diuresis (POD) after decompression had an initial AKI upon admission (63.33%, n=19). The incidence of AKI in the POD group was significantly higher compared to the non-POD group (p=0.004). Similarly, serum creatinine levels were significantly higher in the POD group (p=0.003), with a mean of 1.93 (range: 1.28–3.1). Consequently, those with lower eGFR levels had a significantly higher incidence of POD (p=0.008), with the average eGFR in the POD group being 35.6 mL/min/1.73m² (range: 15.7–58.6).

Serum sodium levels also differed significantly between the POD and non-POD groups (p=0.008). Although still within normal range, baseline sodium levels were higher in the POD group (mean: 140 mmol/L, range: 136–142) compared to the non-POD group (mean: 136.5 mmol/L). There was no significant difference in serum potassium levels between the two groups, with mean values of 3.94 in the POD group and 4.03 in the non-POD group. Ionized calcium levels were similar in both groups, though only 20 records had baseline ionized calcium levels.

Blood urea nitrogen (BUN) was not routinely measured at baseline in obstructive uropathy patients, with only 11 patients having reference BUN levels upon admission. Among those with available results, BUN levels were not significantly different between the POD and non-POD groups (p=0.736).

For a limited number of patients with arterial blood gas on admission, no significant differences in pH (p=0.468) or bicarbonate levels (p=0.434) were observed. The average arterial pH for POD patients was 7.42 ± 0.08, while the POD group had lower serum bicarbonate levels (18.36 ± 6), though this was not statistically significant (p=0.434).

Urine pH and specific gravity were also assessed. Urine pH was more alkaline in those who developed POD (mean: 6.58 ± 0.92), but this was not statistically significant (p=0.278). No significant differences in urine specific gravity (p=0.322) or urine albumin (p=0.928) were found between the groups.

PREDICTORS OF POST-OBSTRUCTIVE DIURESIS (POD)

To identify potential predictors of POD, associations between clinical and renal profiles were analyzed. Age did not significantly correlate with POD risk (OR 0.99, 0.97–1.03; p=0.967). Gender was also not a significant risk factor for developing POD (p=0.176).

Among various diagnoses, only obstruction secondary to prostate cancer was significantly associated with POD. Patients with prostate cancer who had obstruction were 5.2 times more likely to develop POD after an intervention (p=0.011). Other diagnoses, such as benign prostatic hypertrophy, lithiasis, and ureteral stricture, were not significantly associated with POD.

The number of days of obstruction was also a significant predictor of POD. For each additional day of obstruction, the odds of developing POD after obstruction relief increased by 1% (p=0.042). No specific co-morbid conditions or body mass index (BMI) classifications were found to be significantly associated with increased POD risk.

This analysis suggests that AKI, lower eGFR, higher serum sodium levels, longer duration of obstruction, and prostate cancer are important factors in predicting POD. However, other clinical and biochemical markers, such as potassium, calcium, and BUN levels, were not found to be significant predictors in this cohort.

Table 3. Risk Factors Associated with Post-Obstructive Diuresis (POD)

^a includes nephrolithiases (such as staghorn calculi), urolithiases in any part of the urinary tract

^b genitourinary tuberculosis, neurologic pathologies (such as neurogenic bladder, DM cystopathy), blood clots and pelvic inflammatory disease among females

^c pertains to the number of days from the time of obstructive symptoms until any interventions to remove obstruction

^d any smoking history (not specific with pack years)

^edefined by the KDIGO guidelines of Acute Kidney Injury and Chronic Kidney Injury

^f based on the Asia Pacific Classification of Body Mass Index (BMI).

Table 4. Renal Factors Associated with Post-Obstructive Diuresis (POD)

^a serum levels upon admission and prior to decompression

^b based on the Acute Kidney Injury criteria of the KDIGO

The renal profile was analyzed for possible predictors of POD. Table 3 shows the different renal parameters and their association with the development of POD.

The study showed that for every unit that rises in the level of eGFR, the odds of having POD decreased by 2% (p=0.009). The levels of serum sodium, potassium, ionized calcium, and urea, although significantly higher in POD, could not be used to predict the development of POD. Arterial blood gas and urinalysis could not be used to determine any risk factors for having POD.

A baseline acute kidney injury was significantly associated with subsequent POD after a decompression (p=0.004). Obstructive uropathy/nephropathy patients without AKI had 73% the odds of not having POD compared to patients with AKI. Hence, an acute kidney injury increases the likelihood of the patient consequently developing POD. On the other hand, no significant association was determined between chronic kidney disease and POD.

TYPES OF INTRAVENOUS FLUID AND POST-OBSTRUCTIVE DIURESIS

All of the patients whose charts were reviewed had an intravenous fluid (IVF) even before an intervention, and these IVF were maintained during the episodes of polyuria.

Table 5. Choice of Intravenous Fluids among Obstructive Uropathy Patients

*categories based on available data upon chart review

PNSS plain normal saline solution; D5NSS 5% dextrose in normal saline solution; PLRS plain lactaded Ringer’s solution; D5NM 5% dextrose in Normosol-M solution

The most common choice of intravenous fluid among patients admitted for obstructive uropathy or nephropathy was plain NSS, with the rest of the IVF comprising only 16% of the total cases. The second most commonly used IVF was the lactated ringer’s solution for both groups. But, there were no significant differences regarding the choice of IVF between the two groups (p=0.32).

The study specifically aims to identify if there is a significant difference between those who developed POD with PNSS as IVF upon admission and those who were started on PLRS. However, there was no significant difference between the two groups. Also, the study showed that the choice of IVF did not significantly predict POD (P=0.285).

Table 6. Correlation between Intravenous Fluid  ^a  and the Patients’ Course

^a IVF used for analysis was plain normal saline solution

Among patients who developed POD, IVF was continued, adjusted, and modified according to the levels of repeat serum electrolytes. The length of hospital stay and days with polyuria were analyzed against the choice of intravenous fluid, whether or not IVF alters the previous or not. The use of normal saline solution was inversely related to the length of hospital stay (r -0.2502), but the association level was weak. It also showed a longer day with polyuria, but the association was directly weak (r 0.1416 ). The choice of IVF, therefore, did not significantly alter the course of the patients with POD.

Table 7. Correlation between Choice of IVF and Length of Hospital Stay

There was no significant difference between the choice of intravenous and the length of hospital stay (p=0.452), as shown in Table 6. On average, most of the IVF use in POD patients had a length of seven hospital days. In this light, IVF could not be used to predict hospital stays or help decrease the course in the ward.

Intravenous fluids were also studied to determine whether using a specific type would decrease the incidence of electrolyte imbalance during ongoing management of POD. Table 7 shows us the rate of electrolyte imbalances after decompression and with the use of specific intravenous fluids. None of the patients had a significant change in serum sodium levels with any of the IVFs. However, some patients developed hypokalemia post-procedure or postoperatively while ongoing polyuria.

Table 8. Correlation between Choice of IVF and Potassium Levels  ^a

^a Normal level defined as 3.5-5.5 mmoL/L

^b Use of Hypertonic Solution such as 3% NaCl or incorporation of at least 50 mEqs NaCl in the IVF

Most patients did not have a baseline hypokalemia upon admission. For those who presented with baseline hypokalemia, normal saline solution was still the initial intravenous fluid of choice (15.48%,  n=13  ). None of those hydrated with plain Lactated Ringer’s solution had an initial low potassium level. The correlation on whether the use of specific types of IVF among POD patients could not be done since most of the patients did not have repeat serum electrolytes, hence lacking data for comparison. Nevertheless, the levels of serum electrolytes postoperatively among POD patients will be discussed in the section below.

FLUID REPLACEMENT

In some of the patients, additional fluid was given on top of the intravenous fluid (IVF). Options included ORS-75 or pLRS/PNSS/0.45% replacement via intravenous route. A total of fifteen patients were given additional fluid replacement. 86% of these were replaced with pLRS using a specific formula: Volume replacement of pLRS (mL) = [Urine output above 200mL (mL) x 0.75]. None of the patients who were given this kind of replacement developed imbalances in sodium and potassium.

Table 9. Correlation between Fluid Replacement and Hospital Stay

The course of patients with POD did not vary significantly with or without fluid replacement as mentioned above. There may seem to be a significant difference in the days of polyuria between those with and without additional fluid replacement, this difference may be attributed in that most patients given added fluids were already on prolonged polyuria prior to initiation. It, too, did not shorten nor prolong the day of hospital stay as compared to those who only had intravenous fluid for replacement.

CORRELATION BETWEEN POD AND INTERVENTIONS

As mentioned in the prior sections, several interventions could be done to relieve the obstruction. In the majority of the patients, only insertion of an indwelling catheter was needed to relieve the obstruction. This intervention, however, is not statistically significant to correlate to POD in the later course in the ward.

Table 10. Correlation between POD and Decompressive Techniques

Acronyms: PCN—Percutaneous Nephrolithotomy, DJ—Double J, TURP—Transurethral Resection of Prostate

Significantly, patients who had obstruction and underwent Transurethral Resection of Prostate (TURP) had a higher chance of having post-obstructive diuresis (4.87, 1.08-21.85) as compared to other interventions (p=0.039). This is compatible with the initial finding of a significant association between those who had obstruction secondary to prostate malignancy and the development of POD.  

The average time (hours) of surgery of patients who had POD is 1 (0-2) and is significantly longer than those without POD, whose average hours of OR is 0.3 (p=0.009). Likewise, the procedure's duration is a significant risk factor for the development of the POD (p=0.021). For every hour increase in OR, the odds of having POD increase by 64%.

RENAL OUTCOME OF POST-OBSTRUCTIVE DIURESIS

There was a significant improvement in serum creatinine levels after the procedure (p=0.001). There was also an expected increase in the eGFR (p=0.966).

Table 11. Comparison of Outcomes between POD and Non-POD Groups

^a serum levels upon admission and prior to decompression

^b based on CKD-EPI formula

Table 10 shows that there is no significant difference between the baseline and post-operative values of serum sodium, potassium, and ionized calcium. Looking at hospital courses, patients with POD have significantly longer hospital stays compared to the non-POD cohort (p=0.004), with the former averaging to about 8 (6-12) days.

None of the POD patients had a further elevation of serum creatinine or decline in eGFR post-procedure.

Table 12. Incidence of Electrolyte Imbalance in POD after an Intervention

There were only a total of five patients who had hypokalemia in the POD group, and this is not significantly different from those without diuresis. None of the POD patients had hypernatremia. Instead, most had hyponatremia after days with polyuria after the removal of the obstruction. However, P-values showed that post-obstructive diuresis did not lead to a significant electrolyte imbalance compared to those without POD.

All of the patients diagnosed with post-obstructive diuresis improved and were discharged stable and with resolution of the polyuria. Only one had an emergency renal replacement therapy, but it was only acute and was subsequently off hemodialysis. Except for prolonged hospital stays, no other morbidities were noted.

Discussion

Post-obstructive diuresis (POD) refers to the prolonged urine production of at least 200 mL per hour for two consecutive hours following the relief of urinary retention or obstruction, such as with a Foley catheter, nephrostomy, or double-J stent for ureteric obstructions.[9]  The incidence of POD remains unclear, but estimates range from 0.5-52% after decompression.[1]  This study found an incidence of 28.3%, with a male predominance and an average age of 58 years. Many affected patients had co-morbidities such as smoking, acute kidney injury (AKI), chronic kidney disease (CKD), and were overweight.

Common causes of obstruction leading to POD include phimosis, prostatic hyperplasia, meatal stenosis, urinary stones, posterior urethral valves, blocked Foley catheters, urological cancers (e.g., prostate cancer), fecal impaction, diabetes, and neurogenic disorders.[2]  Among the reviewed records, 33% of patients with POD had urolithiasis (including bilateral staghorn calculi). The most significant association was with prostate malignancy, which increased the risk of POD by 5.2%. No previous studies have linked prostate cancer directly to POD.

Patients with POD had significantly longer obstruction durations, averaging 14 days, compared to 7 days in non-POD cases. The risk of POD increased with obstruction lasting two weeks or more. A 2018 case by Shankel documented a pediatric patient developing massive POD after one week of obstruction, a rare finding in the literature. [16] 

The most common intervention for relieving obstruction was Foley catheter insertion, although a significant association was found between transurethral resection of the prostate and POD, likely due to the higher incidence of prostate cancer in the POD group. This suggests a relationship between prostate cancer and the development of POD.

Several mechanisms have been proposed to explain POD, but none have been conclusively proven. One hypothesis involves impaired renal concentration and decreased sodium reabsorption after prolonged obstruction. This may lead to electrolyte imbalances due to leakage of potassium, magnesium, and phosphorus.[9]  A study by Halbgewachs and Domes (2015) proposed a combination of factors, including reduced medullary concentration gradients, decreased glomerular filtration rate, and impaired response to antidiuretic hormone, as causes of POD.[1]  Our study did not find significant differences in specific gravity or urine pH between POD and non-POD groups, suggesting that the kidneys’ ability to concentrate or acidify urine is not a primary factor.

Another theory is that POD arises from the pathophysiology of acute renal failure, which includes elevated serum creatinine, uric acid, hyperkalemia, and metabolic acidosis. [9]  Patients presenting with AKI before decompression had a significantly higher risk of developing POD, as evidenced by higher serum creatinine levels and lower eGFR in the POD group. A similar study by Hamdi identified high serum creatinine, sodium bicarbonate levels, and urinary retention as independent risk factors for POD after decompression.[8]  Our study confirmed that higher serum creatinine levels (mean 1.93 mg/dL) were associated with an increased risk of POD, whereas patients without POD had lower creatinine levels (mean 1.2 mg/dL). Lower baseline eGFR was also a predictor of POD, although bicarbonate levels did not show a significant correlation with POD.

Serum sodium levels were notably higher in the POD group, with an average of 140. This suggests that sodium balance may be disrupted in these patients, though it was not correlated with the severity of POD. Identifying patients at higher risk for POD allows for better prevention of complications like severe dehydration, electrolyte imbalances, and hypovolemic shock, which, if untreated, can be fatal.[9] 

The average duration of polyuria in POD patients was 96 hours, significantly longer than the 48-hour diuretic phase observed in other studies such as Harrisons et al.[9]  Despite intravenous fluid (IVF) and electrolyte replacements, this phase remains critical for managing dehydration and potential renal injury. Monitoring fluid balance and electrolytes is essential during this phase. [1,2,4,5,7] 

IVF management typically involves replacing urine output above 200 mL per hour, with most patients receiving lactated Ringer's solution (pLRS). One study showed that this approach significantly reduced the incidence of electrolyte imbalances, including hypokalemia.

Most fluids administered were normal saline (0.9%), although other IVF types, such as lactated Ringer’s, D5NM, and hypertonic solutions, were also used. The choice of IVF did not significantly impact the course of POD or length of hospital stay. Notably, patients who received pLRS showed no significant electrolyte imbalances despite prolonged polyuria, unlike those receiving other fluids. This could explain the lower incidence of hypokalemia and acute kidney injury in the POD group. Excessive fluid replacement, however, might prolong diuresis, as indicated in some studies.

Hospital stay was prolonged for POD patients, with an average of 8 days compared to 5 days for non-POD patients. This extended stay may contribute to increased healthcare costs. Despite this, POD resolution led to improvements in eGFR and reductions in serum creatinine, with no significant electrolyte imbalances. Importantly, there were no deaths related to POD in this cohort.

In summary, factors such as longer obstruction duration, prostate cancer, acute kidney injury, and lower baseline eGFR were identified as significant risk factors for POD. These findings can help predict which patients are at greater risk, allowing for more targeted interventions. By monitoring fluid balance, electrolytes, and renal function closely, complications from POD can be minimized, improving patient outcomes and reducing the associated healthcare burden.

Conclusion

The incidence of post-obstructive diuresis was 28.3%, with a mean age of 58 years and with male predominance. The most frequently associated co-morbid conditions include smoking, acute kidney injury, and chronic kidney injury. There were also significant proportions of overweight patients in the POD group. Patients with POD had the following significant baseline renal profiles compared to the non-POD group: higher levels of serum creatinine, higher levels of serum sodium, and lower eGFR values. Risk factors associated with POD included a baseline acute kidney injury, relief of obstruction from prostate cancer, and longer days of obstruction. Decompression via transurethral resection of the prostate was significantly associated with POD, and the longer duration of OR increases the risk of it as well. There was no significant difference between the use of different IVF and the length of hospital stay, days of polyuria, and electrolyte imbalance. This is true with additional fluid replacement with pLRS as well. In terms of outcome, all patients improved with hydration. There were no significant electrolyte imbalances; however, the hospital stay was longer.

LIMITATIONS AND RECOMMENDATIONS

Although the study population is comparable to recent and popular works of literature on POD, the study still needs a population for better power. Hence, we recommend a more extensive review of charts by extending the involved years. Also, since it is a relatively rare entity, a prospective study could be done in order to better follow the course and also to be able to work the patient up appropriately.