What is the significance of urea plasma in urine testing?

Significance of Urea in Urine Testing

Urea measurement in urine is clinically significant for assessing residual kidney function, calculating dialysis adequacy, differentiating causes of acute kidney injury, and predicting chronic kidney disease progression.

Assessment of Residual Kidney Function in Dialysis Patients

Urine urea clearance is essential for calculating total small-solute clearance (Kt/V) in dialysis patients, which directly impacts survival. 1

• In hemodialysis patients, residual kidney urea clearance (Kru) should be measured quarterly when urine volume exceeds 100 mL/day, as it significantly contributes to total dialysis adequacy 1
• For peritoneal dialysis patients with urine volume >100 mL/day, 24-hour urine collections for urea clearance should be obtained at minimum every 2 months 1
• Residual kidney function is a stronger predictor of survival than peritoneal clearance alone, with each 1 mL/min/1.73 m² increase in residual kidney function associated with a relative risk of 0.41 for mortality 1
• Urine volume itself is an independent predictor of survival, with each 100 mL/24-hour increase associated with a 10% reduction in mortality risk 1

Fractional Excretion of Urea (FEUrea) in Acute Kidney Injury

FEUrea is superior to fractional excretion of sodium (FENa) for differentiating hepatorenal syndrome from acute tubular necrosis, particularly in patients receiving diuretics. 1

• FEUrea <28.16% has 75% sensitivity and 83% specificity for diagnosing hepatorenal syndrome versus other causes of acute kidney injury 1
• Unlike FENa, FEUrea is not affected by diuretic use because urea is primarily reabsorbed in the proximal tubule and collecting ducts 1
• In cirrhotic patients, FENa <1% has 100% sensitivity but only 14% specificity for prerenal causes, making it unreliable in this population 1

Urine-to-Plasma Urea Ratio as a Prognostic Marker

The urine-to-plasma (U/P) urea ratio serves as an early marker of tubular dysfunction and predicts chronic kidney disease progression, particularly in patients with eGFR ≥30 mL/min/1.73 m². 2, 3

• Each 1-SD decrease in U/P urea ratio is independently associated with a 22% increased risk of CKD progression (HR 1.22,95% CI 1.12-1.33) 3
• Among patients with eGFR ≥30 mL/min/1.73 m², lower U/P urea ratios predict 30% higher risk of progression (HR 1.30,95% CI 1.18-1.45) 3
• The U/P urea ratio reflects urine-concentrating capacity and tubular function, providing information beyond glomerular filtration rate alone 2, 3
• This marker is particularly valuable because urea measurement is standardized, widely available, and inexpensive 2

Calculation of Protein Catabolic Rate

Urine urea nitrogen is essential for accurately calculating protein catabolic rate (PCR) in dialysis patients, which assesses nutritional status. 1

• When significant residual kidney urea clearance is not included in kinetic modeling, PCR will be significantly underestimated 1
• Accurate PCR calculation requires 24-hour urine collection combined with pre- and post-dialysis blood samples 1

Clinical Monitoring Recommendations

For dialysis patients with residual function, urine volume should be monitored monthly, with formal urea clearance measurements every 2-4 months. 1

• Unscheduled measurements are indicated when urine volume changes abruptly, after hospitalization, or with clinical deterioration 1
• Measurements should be deferred until at least 1 month after peritonitis resolution, as acute illness transiently alters clearance 1
• In stable chronic kidney disease patients not on dialysis, monitoring every 2-4 months is appropriate 4

Common Pitfalls to Avoid

• Do not rely solely on blood urea nitrogen (BUN) levels, as both high and low levels can indicate poor outcomes depending on context 4
• Avoid measuring clearances during acute illness or within 1 month of peritonitis, as results will not reflect stable baseline function 1
• Do not assume FENa is reliable in cirrhotic patients or those on diuretics—use FEUrea instead 1
• Remember that U/P urea ratio loses predictive value in advanced CKD (eGFR <30 mL/min/1.73 m²), so it is most useful for earlier-stage disease 3