Will labs be unstable in a patient with impaired renal function undergoing dialysis?

Laboratory Stability Post-Dialysis

Yes, laboratory values are expected to be unstable immediately following dialysis treatment, with the postdialysis blood urea nitrogen (BUN) representing a "moving target" that sharply reverses when treatment ceases, making timing of blood sampling critical for reliable measurements. 1

Immediate Post-Dialysis Laboratory Changes

BUN and Creatinine Dynamics

• Postdialysis BUN concentrations increase sharply after dialysis cessation, requiring standardized timing of blood sample collection to achieve reproducible values 1
• Blood samples should be drawn using the slow flow/stop pump technique at the end of dialysis to ensure uniformity and reliability of Kt/V measurements 1
• The blood reinfusion sampling technique yields systematically lower Kt/V values compared to the slow flow/stop pump method, even when the delivered dialysis dose is identical 1

Glucose Fluctuations

• Frequent hypoglycemia occurs during and after hemodialysis, with higher glucose levels typically observed during early morning the day after dialysis 1
• Glucose levels reach a nadir during the third hour post-hemodialysis 1
• Capillary glucose testing has substantial accuracy limitations in dialysis patients, with mean absolute relative differences ranging from 5.6% to 20.8% 1

Electrolyte and Volume Status

• Laboratory assessments should occur within 3 days (and no later than 7 days) after the last dialysis session for accurate evaluation 1
• Intradialytic hypotension and excessive ultrafiltration cause hemodynamic instability that affects laboratory parameters 2

Factors Contributing to Laboratory Instability

Urea Rebound Phenomenon

• Urea clearance measurements are affected by the timing of postdialysis sampling because urea continues to redistribute from tissues after dialysis stops 1
• Markedly slowing blood flow at the end of dialysis before sampling is the safest technique for achieving reliable values 1

Muscle Mass and Malnutrition Effects

• Low postdialysis creatinine levels indicate decreased skeletal muscle mass and/or low dietary protein intake, both associated with increased mortality 3
• Assessment of kidney function must account for loss of muscle mass and its impact on serum creatinine as a GFR marker 1
• Alternative markers like cystatin C or direct GFR measurement with iohexol clearance should be considered in selected cases 1

Hemoglobin A1c Inaccuracy

• HbA1c measurements have low accuracy in advanced chronic kidney disease due to reduced erythrocyte lifespan, anemia, transfusions, and use of erythrocyte-stimulating agents 1
• Metabolic acidosis and elevated urea nitrogen lead to formation of carbamylated hemoglobin, causing high bias in HbA1c measurements 1

Clinical Implications for Laboratory Interpretation

Timing Recommendations

• Predialysis and postdialysis BUN samples for a given patient should be processed in the same batch analysis to minimize interassay variability of approximately 3% 1
• Measurement of serum urea, creatinine, and electrolytes should occur at least every 48 hours or more frequently if clinically indicated 2

Standardization Requirements

• Hemodialysis facilities should adopt a single BUN sampling method and record it routinely, maintaining consistency for individual patients to permit longitudinal analysis 1
• The sampling method used for a given patient should remain consistent over time 1

Common Pitfalls to Avoid

• Do not rely on single postdialysis laboratory values without considering the timing of sample collection 1
• Avoid comparing laboratory values between facilities using different sampling techniques, as apparent performance differences may be due solely to sampling method 1
• Do not use BUN alone to monitor kidney function, particularly in diabetic patients, as it may remain low due to decreased protein intake despite significant renal impairment 3

Special Considerations for Residual Kidney Function

Monitoring Requirements

• Regular assessment of residual kidney function using 24-hour urine collection should include volume of urine output as well as creatinine and urea clearance 1
• Weekly assessment of serial pre-dialysis serum creatinine values is recommended for patients discharged while still receiving dialysis 1

Impact on Laboratory Interpretation

• Residual kidney function is more closely associated with better outcomes than the dialysis dose itself 1, 4
• Patients with residual urea clearance greater than 2 mL/min should have regular measurements to avoid under-dialysis as function is lost 1