How should I interpret a 24‑hour urine collection in an end‑stage renal disease patient?

Interpreting 24-Hour Urine Collections in ESRD Patients

In ESRD patients on dialysis, 24-hour urine collections are used primarily to measure residual kidney function (RKF) through urine volume and clearance calculations, which directly impacts dialysis prescription, volume management, and survival outcomes.

Primary Purpose: Assessing Residual Kidney Function

When to Measure

For peritoneal dialysis patients:

• Measure RKF within the first month after initiating dialysis, then at least every 4 months thereafter 1
• If urine output exceeds 100 mL/day and RKF contributes to total clearance goals, obtain 24-hour collections every 2 months minimum 1
• Once weekly Kt/V falls below 0.1, RKF is negligible and routine measurement can stop 1

For hemodialysis patients:

• No consistent consensus exists on measurement frequency; it is measured far less routinely than in PD patients 1
• Collection timing should ideally span the entire interdialytic period 1

What to Measure

Calculate residual GFR using:

• The mean of urea and creatinine clearance from 24-hour urine collection with simultaneous blood sampling 1
• This method is standard for peritoneal dialysis patients 1

For hemodialysis patients:

• Some centers measure only urine volume rather than performing full clearance calculations 1
• Research shows residual urine volume alone is a stronger predictor of mortality than calculated GFR in dialysis patients 2

Clinical Significance and Interpretation

Urine Volume Thresholds

Critical cutoff: 100 mL/day

• Urine output >100 mL/day indicates significant RKF that should be incorporated into dialysis adequacy calculations 1
• Urine output ≤100 mL/day defines anuria; RKF is considered negligible for prescription purposes 1
• Anuric patients show significantly higher fluid overload compared to those with urine output >100 mL/day 3

Clearance Values

For peritoneal dialysis:

• Total weekly Kt/Vurea (residual kidney + peritoneal) must be ≥1.7 1
• RKF typically decreases at approximately 1 L/week/1.73 m² per month (0.1 mL/min/month) 1
• By 37 months, average kidney Kt/Vurea falls below 0.1 in most patients 1

Prognostic Value

Higher residual urine volume independently predicts:

• Lower all-cause mortality (hazard ratio 0.96 per 0.1 L/day higher volume) 2
• Better volume status and reduced fluid overload 3
• Lower interdialytic weight gain and fewer hypotensive episodes 1

Collection Technique Considerations

Standard Protocol

For patients voiding >3 times per 24 hours:

• Standard 24-hour collection is adequate 1

For patients voiding ≤3 times per 24 hours:

• Extend collection to 48 hours to avoid sampling errors 1

Verification of adequacy:

• Measure 24-hour creatinine excretion simultaneously to confirm complete collection 1
• Without creatinine verification, protein and clearance estimates are often incorrect 4

Pre-Collection Instructions

• Avoid vigorous exercise for 24 hours before collection 4
• Avoid collection during acute illness, marked hyperglycemia, marked hypertension, or heart failure 4
• Discard first morning void at start time, then collect all subsequent urine for exactly 24 hours, including final void 4

Impact on Dialysis Prescription

Peritoneal Dialysis Adjustments

When RKF is significant (>100 mL/day):

• Include residual kidney Kt/Vurea in total clearance calculation 1
• Adjust peritoneal dialysis prescription to achieve combined target of ≥1.7 weekly Kt/Vurea 1
• Monitor every 2 months due to variable rate of RKF decline 1

When RKF becomes negligible (<100 mL/day):

• Peritoneal Kt/Vurea alone must be ≥1.7 per week 1
• Increase dialysis prescription to compensate for lost kidney function 1

Hemodialysis Considerations

• Preservation of residual urine volume is an important treatment goal 3
• Volume control through adequate dialysis and sodium restriction helps optimize outcomes 5
• Avoid intradialytic hypotension to preserve RKF 1

Common Pitfalls

Do not:

• Rely on serum creatinine alone to assess kidney function in ESRD patients, as it does not reflect residual clearance 6
• Assume anuric patients have zero kidney function without measuring; even small amounts of RKF improve outcomes 2
• Delay measurement until symptoms develop; RKF declines steadily and requires proactive monitoring 1
• Use estimated GFR equations in ESRD patients; they are unreliable at very low GFR levels 4

Critical monitoring:

• In PD patients with declining urine output, increase monitoring frequency to every 2 months to prevent falling below minimum clearance targets 1
• Measure both volume and clearance when RKF contributes to adequacy goals; volume alone is insufficient for prescription adjustments 1

Strategies to Preserve Residual Kidney Function

Evidence-based interventions:

• Use RAS blockers (ACE inhibitors or ARBs) 1
• Avoid nephrotoxins and intradialytic hypotension 1
• Consider loop diuretics to maintain urine volume and reduce ultrafiltration requirements 1
• In PD patients, use neutral pH, low glucose degradation product solutions 1