How is the efficacy of dialysis measured?

How to Check Efficacy of Dialysis

Primary Measurement Method

The delivered dose of dialysis should be measured using Kt/V (where K = dialyzer urea clearance, t = time, V = patient's urea distribution volume), with measurements performed at least monthly for all hemodialysis patients. 1

Hemodialysis Adequacy Assessment

Core Measurement: Kt/V Calculation

• Single-pool Kt/V (spKt/V) is the preferred measure, calculated through formal urea kinetic modeling from pre- and post-dialysis blood urea nitrogen (BUN) samples 1
• The minimum target spKt/V is 1.2 per dialysis session for thrice-weekly treatments 1
• Equilibrated Kt/V (eKt/V) accounts for post-dialysis urea rebound and provides a more accurate assessment, with a minimum target of 1.0 1

Blood Sampling Technique

• Both pre- and post-dialysis BUN samples must be drawn during the same treatment session and analyzed together to minimize interassay variability 1
• Pre-dialysis sample: Draw immediately before starting dialysis to avoid saline dilution 1
• Post-dialysis sample: Slow blood flow to 100 mL/min for 15 seconds before sampling to avoid access recirculation, which would falsely lower the BUN and overestimate dialysis adequacy 1
• Alternative method: Stop dialysate flow for 3 minutes before obtaining the post-dialysis sample 1

Measurement Frequency

• Monthly measurements are mandatory for quality assurance 1
• Increase measurement frequency when: patients miss treatments, blood flow problems occur, wide variability in results appears, or prescriptions are modified 1

Alternative Measurement Methods (in order of preference after Kt/V)

• Urea reduction ratio (URR): Simpler calculation but less precise than Kt/V 1
• On-line ionic clearance or dialysate urea monitoring: Provides continuous, bloodless measurements 1
• Total dialysate collection with urea mass measurement: Gold standard but impractical for routine use 2

Peritoneal Dialysis Adequacy Assessment

Weekly Clearance Targets

• Total weekly Kt/V of at least 2.0 for CAPD patients 1
• Total creatinine clearance of at least 60 L/week/1.73 m² for high/high-average transporters and 50 L/week/1.73 m² for low/low-average transporters 1

Collection Method

• Collect all dialysate effluent over 24 hours with simultaneous timed urine collection 1
• Use the aliquot method: From each effluent bag, collect a volume in mL equal to the bag volume divided by 1000 (e.g., 2.45 mL from a 2,450 mL bag) 1
• Measure urea and creatinine concentrations in pooled dialysate and urine samples 1

Residual Kidney Function Assessment

• Measure residual kidney urea clearance (Kr) from timed urine collection at regular intervals 1
• Include Kr in adequacy calculations, as it provides continuous clearance between dialysis sessions and improves patient outcomes 1
• Urine output alone does not accurately predict residual kidney function and should not be used as the sole determinant 1

Nutritional Status Assessment

• Calculate normalized protein catabolic rate (nPCR) from urea generation rate, which should be 1.0-1.2 g/kg/day 1
• For peritoneal dialysis, use the modified Borah equation accounting for dialysate protein losses 1
• Monitor nPCR longitudinally as declining values may indicate malnutrition despite adequate Kt/V 1

Critical Pitfalls to Avoid

• Clinical symptoms alone are unreliable for assessing dialysis adequacy; quantitative measurements are essential 1
• Access recirculation during post-dialysis sampling will falsely elevate Kt/V by 10-20%, leading to undetected underdialysis 1
• In malnourished patients, Kt/V may appear adequate while the patient remains underdialyzed, as lower protein intake reduces urea generation 1
• Failure to measure residual kidney function regularly may lead to unnecessary continuation of dialysis in patients with recovering renal function 1

Quality Assurance

• Compare delivered dose to prescribed dose monthly to identify problems with vascular access, blood pumps, or dialysate delivery 1
• Monitor dialyzer function through total cell volume (TCV) measurements or ionic clearance, rejecting dialyzers with >10% clearance loss 1
• Track Kt/V trends over time; unexplained decreases warrant investigation even if values remain above minimum targets 1