Platelet loss across the hemofilter during continuous hemofiltration

Background: Thrombocytopenia is a common finding in patients in the intensive care unit receiving continuous renal replacement therapy (CRRT). It is unknown if the hemofilter itself contributes to the platelet loss. Objective: To measure the direct effect of the hemofilter on platelet counts during CRRT. Design: Prospective, observational study. Setting: Intensive care unit of a University hospital. Patients: Critically ill patients with acute renal failure receiving CRRT. Methods: Two samples of blood were drawn simultaneously, pre-filter and post-filter, and analyzed for platelet count. A correction factor was applied to the post-filter platelet count to adjust for the hemoconcentrating effect of net ultrafiltration. Results: Forty-eight sets of paired data from 22 patients were studied. There was a small but significant decrease in mean platelet count across the hemofilter. The mean platelet count drop was 2.32 × 10⁹/L (s.e. 1.06, p=0.0487, 95% CI (0.01,4.62)). Blood flow was strongly related to degree of platelet loss, with a decreased loss of 0.07 × 10⁹/L for every ml/min increase in blood flow (p=0.015). There was no overall decrease in concurrently measured red cell counts across the hemofilter. However, there was a machine-specific affect on red cell loss (p<0.0001). The total calculated daily platelet loss across the filter was 625 × 10⁹ cells. Conclusion: The hemofilter may contribute to the thrombocytopenia seen during CRRT, by means of either destruction or retention of platelets during passage. This affect appears attenuated by higher blood flows. This information is useful in the assessment of a low platelet count in patients receiving CRRT.