Causes of impaired delivery of hemodialysis prescription

Urea kinetic modeling (Kt/V) is used to assess adequacy of hemodialysis (HD). However, serial Kt/V measurements may vary with time in the same patient, making the interpretation of the results difficult. The aim of the present study was to find the frequency and the causes that account for these fluctuations of Kt/V. Fifty-nine patients undergoing chronic HD were included in this prospective study. The results of monthly single pool Kt/V values were analyzed during a 6-month period. Duration of maintenance HD prior to the study was 4.49 +/- 3.6 (+/- SD) years. Any change of > 0.2 from the previous 2-month average values was defined as abnormal. A total of 354 urea kinetic modeling sessions were recorded during 6 months in 59 patients. Of these, 38 (10.7%) met the criteria for abnormal value. Twenty-four measurements (6.7%) revealed lower while 14 (3.9%) demonstrated higher Kt/V values. Supervised sampling and conforming to the prescribed dialysis dose were applied for all abnormal measurements a week later. Among the group with lower Kt/V value, nine were due to noncompliance (shorter dialysis), four lower blood flow (Qb), four reversed needles, and one fistula thrombosis. Finally, in six cases no problem could be identified and a repeat measurement failed to document lower values. In the high-value group, nine cases were expected as there was an effort to increase dialysis dose prescription while five cases were due to false postdialysis sampling ("venous samples"). Overall, 29/354 (8.1%) measurements were in real disagreement with dialysis prescription. Lower-than-expected values are quite often due to reduced blood processing (shorter dialysis, lower Qb, and recirculation) and higher values due to inaccurate postdialysis sampling. Any possible pitfall in Kt/V measurements should be investigated before changing dialysis prescription in a stable HD patient.