Should labs be drawn before or after dialysis in a skilled nursing facility patient with impaired renal function?

Laboratory Timing for Dialysis Patients in Skilled Nursing Facilities

Labs should be drawn both before AND after dialysis, with predialysis samples obtained immediately before dialysis initiation and postdialysis samples collected using the slow flow/stop pump technique immediately after dialysis completion. 1

Predialysis Laboratory Collection

Draw predialysis labs immediately before dialysis begins to accurately reflect the patient's baseline metabolic status and guide treatment decisions. 1, 2

Critical Predialysis Sampling Requirements:

• Obtain the blood specimen before connecting arterial blood tubing or flushing the needle to prevent dilution with saline or heparin, which would artificially lower BUN values and compromise dialysis adequacy calculations. 1

• For arteriovenous fistulas/grafts: Draw from the arterial needle prior to any line connection, ensuring no saline or heparin is present in the needle or tubing. 1

• For venous catheters: Withdraw and discard 10 mL of blood (3-5 mL in pediatrics) after removing heparin/saline to ensure an undiluted sample. 1

• Never draw predialysis samples after dialysis has started, as this will reflect dialyzed blood with artificially reduced solute concentrations, leading to falsely elevated dialysis adequacy measurements (Kt/V, URR) and incorrect volume calculations. 1

Postdialysis Laboratory Collection

Draw postdialysis labs immediately after dialysis ends using the slow flow/stop pump technique to accurately measure dialysis adequacy while minimizing urea rebound effects. 1, 2

Slow Flow/Stop Pump Technique (Preferred Method):

1. At dialysis completion, turn off dialysate flow and reduce ultrafiltration rate to 50 mL/h or minimum setting. 1

2. Decrease blood flow to 50-100 mL/min for 15 seconds to clear the arterial line of any recirculated blood. 1

3. Draw the sample from the arterial sampling port closest to the patient while blood pump runs at 50-100 mL/min (slow flow technique) OR immediately after stopping the pump (stop pump technique). 1

4. This timing is critical: Sampling must occur before significant urea rebound (which begins 2-3 minutes post-dialysis) but after access recirculation has resolved. 1

Why This Timing Matters:

• Drawing postdialysis samples too early (before dialysis ends) increases the postdialysis BUN concentration, underestimating dialysis adequacy. 1

• Drawing samples too late (>5 minutes after dialysis) allows urea rebound from tissue compartments, significantly elevating BUN and again underestimating dialysis adequacy. 1

• The slow flow/stop pump technique provides minimal technical variability between sessions, making longitudinal comparisons of dialysis adequacy more accurate. 1

Monthly Monitoring Requirements for SNF Patients

Measure dialysis adequacy (Kt/V or URR) monthly using properly collected pre- and postdialysis BUN samples from the same dialysis session. 1, 2, 3

Additional Monthly Labs (Predialysis):

• Hemoglobin (more stable and accurate than hematocrit). 3
• Serum ferritin and transferrin saturation to guide iron therapy. 3
• Electrolytes: sodium, potassium, calcium, phosphorus, magnesium, bicarbonate. 3

Quarterly Labs:

• Serum albumin every 3 months as a nutritional marker and mortality predictor. 3

Common Pitfalls to Avoid

Dilution errors are the most common cause of inaccurate dialysis adequacy measurements. 1

• Never flush lines with saline before drawing predialysis samples - this artificially lowers BUN, resulting in falsely elevated Kt/V/URR and incorrect treatment prescriptions. 1

• Avoid drawing pre- and postdialysis samples from different dialysis sessions - this provides meaningless information about delivered dialysis dose due to interassay variability. 1

• Do not use the blood reinfusion technique for postdialysis sampling - this less reproducible method allows excessive urea rebound (taking 5+ minutes), resulting in partially equilibrated samples that don't accurately reflect single-pool kinetic models. 1

• Ensure both samples are analyzed by the same laboratory at the same time to minimize interassay variability, which accounts for most variation in dialysis adequacy measurements (4.0% coefficient of variation for Kt/V). 1

Clinical Context for SNF Patients

SNF dialysis patients have particularly high mortality risk (39.2% at 6 months), making accurate dialysis adequacy monitoring essential for optimizing outcomes. 4

• These patients often have multiple nondisease-specific problems (cognitive impairment, functional dependence, polypharmacy) that increase mortality risk independent of dialysis adequacy. 4

• Proper lab timing ensures accurate Kt/V measurement, allowing clinicians to distinguish between inadequate dialysis delivery versus other causes of poor outcomes in this vulnerable population. 2, 3