Diagnosing Post Urinary Retention Diuresis
Post urinary retention diuresis (PURD) is diagnosed by observing excessive urine output (>200 ml/hour for 2+ consecutive hours) after relieving urinary obstruction, along with monitoring for electrolyte abnormalities and volume depletion. 1
Definition and Pathophysiology
- Post-obstructive diuresis (POD) is characterized by excessive urination following the relief of urinary retention, typically defined as urine output exceeding 200 ml per hour for at least two consecutive hours 1
- The condition occurs in approximately 30% of patients with urinary retention 1
- It results from accumulated fluid and solutes during the period of obstruction, leading to osmotic diuresis once the obstruction is relieved 2
Risk Factors and Predictors
- Residual urine volume >1150 ml at the time of catheterization (sensitivity 84%, specificity 78%) 1
- Elevated serum creatinine >120 μmol/L (sensitivity 68%, specificity 82%) 1
- Increased serum urea levels 1
- Abnormal blood pressure (both systolic and diastolic) 1
- Chronic urinary retention rather than acute retention 3
Diagnostic Approach
Initial Assessment
- Measure urine output hourly after catheterization for at least 24 hours 1, 3
- Calculate fluid balance (intake vs. output) to identify excessive diuresis 1
- Monitor vital signs for signs of volume depletion (tachycardia, hypotension) 3
Laboratory Evaluation
- Serum electrolytes (particularly sodium, potassium) before and after catheterization 2
- Serum creatinine and BUN/urea levels 1
- Urine sodium concentration and osmolality 2
- Urine specific gravity to assess concentration 1
Imaging and Additional Testing
- Renal ultrasound to evaluate for hydronephrosis 3
- Pressure flow studies if needed to determine the cause of the original retention 4
Diagnostic Criteria
- Urine output >200 ml/hour for at least 2 consecutive hours after catheterization 1
- Urine sodium levels and osmolality consistent with dilute urine 2
- Presence of electrolyte abnormalities (particularly hyponatremia) 2
- Evidence of volume depletion (tachycardia, hypotension, decreased skin turgor) 3
Common Pitfalls and Caveats
Misdiagnosis Risks
- Failing to distinguish between appropriate diuresis (accumulated fluid elimination) and pathological diuresis (excessive fluid and electrolyte loss) 2, 3
- Not recognizing hyponatremia that may develop during urinary retention 2
- Overlooking the risk of rapid sodium autocorrection after catheterization, which can lead to osmotic demyelination syndrome 2
Management Considerations
- Avoid administration of hypertonic or normal saline in patients with hyponatremia due to urinary retention, as this can exacerbate rapid autocorrection 2
- Consider hypotonic fluid administration when rapid autocorrection of hyponatremia is occurring 2
- Monitor for decompressive hematuria, another potential complication of severe urinary retention relief 3
Monitoring Protocol
- Hourly urine output measurement for at least 24 hours after catheterization 1, 3
- Serial electrolyte monitoring every 4-6 hours during the acute phase 2, 3
- Fluid balance assessment (intake vs. output) every 8 hours 1
- Hemodynamic monitoring including blood pressure and heart rate 3
- Weight measurements to track fluid status 3
By following this diagnostic approach, clinicians can identify post urinary retention diuresis early and implement appropriate management strategies to prevent complications related to fluid and electrolyte imbalances.