How is urine osmolality used in discontinuing continuous renal replacement therapy (CRRT)?

Role of Urine Osmolality in Discontinuing Continuous Renal Replacement Therapy

Urine osmolality is not currently recommended as a primary parameter for determining discontinuation of continuous renal replacement therapy (CRRT), with urine output being the most validated and reliable predictor of successful discontinuation. 1

Primary Parameters for CRRT Discontinuation

Urine Output

• Most extensively studied and validated parameter for predicting successful RRT discontinuation
• Pooled sensitivity of 66.2% (95% CI, 53.6-76.9%) and specificity of 73.6% (95% CI, 67.5-79.0%) 2
• Area under ROC curve of 0.808 for predicting successful discontinuation 3
• Recommended thresholds:

  • 400 mL/24h without diuretics

  • 2000 mL/24h with diuretics 1

Renal Function Markers

• Creatinine clearance:
  • Independent predictor of successful discontinuation (OR 1.066,95% CI 1.022-1.111) 4
  • Optimal cut-off of 11 mL/min (95% CI 6-16 mL/min) 4
• Creatinine ratio (day 2/day 0):
  • Strong predictor with AUC of 0.819 (95% CI 0.732-0.907) 4
  • Optimal cut-off of 1.41 (95% CI 1.27-1.59) 4
  • Incremental creatinine ratio >1.5 indicates high risk of RRT restart within 90 days 5

Advanced Predictive Models

Kinetic eGFR

• Newly developed method based on dynamic changes of serum creatinine
• Excellent predictor with AUC of 0.87 (95% CI 0.73-0.94) 6
• Combined with urine output provides even better prediction (AUC 0.93,95% CI 0.82-0.97) 6

Multivariate Approach

• Combining parameters improves predictive accuracy:
  • Urine output + creatinine trends + non-renal SOFA score 4
  • Kinetic eGFR + urine output 6

Clinical Implementation Algorithm

1. Primary assessment: Evaluate urine output over 24 hours

   • Without diuretics: Target >400 mL/24h
   • With diuretics: Target >2000 mL/24h (note: diuretics reduce predictive value)

2. Secondary assessment: Evaluate renal function

   • Calculate creatinine clearance (target >11 mL/min)
   • Calculate creatinine ratio day 2/day 0 (target <1.41)
   • If available, calculate kinetic eGFR

3. Additional factors to consider:

   • Duration of CRRT (shorter duration associated with successful discontinuation) 7
   • Non-renal SOFA score (lower score favors successful discontinuation) 4
   • Cumulative fluid balance (lower balance favors successful discontinuation) 4

4. Post-discontinuation monitoring:

   • Daily urine output
   • Creatinine trends (particularly the day 2/day 0 ratio)
   • Need for vasopressors (continued need reduces success likelihood)

Important Caveats

• The predictive ability of urine output is significantly reduced when diuretics are used (AUC 0.671 with diuretics vs. 0.845 without diuretics) 3
• Failed CRRT discontinuation is associated with higher hospital mortality (42.7% vs. 28.5%) 3
• Patients with incremental creatinine ratio >1.5 at day 2 after discontinuation should receive close nephrological follow-up 5
• There is significant heterogeneity in the literature regarding optimal urine output thresholds 2

While various parameters have been studied, urine osmolality is notably absent from major guidelines and research as a primary parameter for CRRT discontinuation decisions, with the focus instead being on urine output, creatinine clearance, and creatinine ratio trends.