What are the tracer dialysis readings?

From the Guidelines

To ensure accurate tracer dialysis readings, it is crucial to follow a standardized protocol for collecting blood and dialysate samples, considering factors that may compromise urea clearance and treatment time, as outlined in the NKF-K/DOQI clinical practice guidelines for hemodialysis adequacy 1. When collecting blood samples for tracer dialysis readings, it is essential to consider the timing and handling to avoid errors that could affect the results.

• Collect blood samples at specific intervals during dialysis treatment, typically at the start, midpoint, and end of the session.
• Use standardized collection tubes and immediately process samples according to laboratory protocols.
• For urea kinetic modeling, blood samples should be drawn pre-dialysis, post-dialysis (within 2 minutes of treatment completion), and 30 minutes after dialysis to account for urea rebound. The dialysate should be collected using the proportional sampling method throughout the entire treatment, as this helps calculate key parameters like Kt/V and urea reduction ratio (URR), which indicate dialysis adequacy 1. Accurate tracer dialysis readings are vital because they guide treatment adjustments to ensure patients receive sufficient toxin removal, directly impacting clinical outcomes and quality of life.
• Errors in collection timing or handling can significantly affect results and lead to inappropriate treatment modifications.
• Factors that may compromise urea clearance, such as access recirculation, inadequate blood flow, and dialyzer clotting, should be considered when interpreting tracer dialysis readings 1.
• Reductions in treatment time, due to inaccurate assessment of effective time or incorrect assumption of continuous treatment, can also impact the accuracy of tracer dialysis readings 1.

From the Research

Tracer Dialysis Readings

• Tracer dialysis readings are crucial in assessing the effectiveness of dialysis treatments, including continuous ambulatory peritoneal dialysis (CAPD) and hemodialysis (HD) 2, 3.
• The clearance of small solutes, such as urea, is an important factor in determining the adequacy of dialysis, with CAPD patients occasionally developing uremic symptoms that can be reversed with increased exchange volume or frequency 2.
• Mathematical kinetic models have been used to compare urea removal in HD and CAPD, demonstrating that the time-averaged urea-nitrogen concentration in the blood of a patient on maintenance HD can be calculated and related to the constant urea-nitrogen concentration in the blood of a patient on CAPD 3.

Kt/Vurea Calculations

• Kt/Vurea is a method of measuring dialysis adequacy that takes into account the clearance of urea, treatment time, and volume of urea distribution 4, 5.
• Different formulae are available for calculating Kt/Vurea, and the appropriate formula to use depends on various parameters, including patient size, frequency of dialysis, and residual kidney function 4.
• The peak concentration hypothesis suggests that uremic toxicity is related to the peak rather than the time-averaged urea concentration, and that a higher Kt/V would be required in HD to achieve a peak concentration at or below the steady-state CAPD concentration 5, 6.

Optimizing Urea Clearances

• Recent studies have suggested that the relationship between the net normalized protein catabolic rate and weekly clearance of urea normalized to total body water is curvilinear rather than linear 6.
• The curvilinear relationship seems likely, with the nPNA plateau achieved at lower Kt/V in patients on CAPD than in those on HD, compatible with the peak concentration hypothesis 6.
• Weekly Kt/V urea requirements to achieve nPNA values at 95% of the asymptote are greater than those usually delivered, but optimum therapy may be well above adequate therapy relative to minimizing appetite suppression by uremia 6.