Falsely increased HCG in patients with high leukocyte counts.

Falsely increased plasma troponin (Tn) I and T results have been reported for several Tn methods, including the Beckman Coulter UniCel DxI AccuTnI assay (Beckman Coulter, Chaska, MN) (1–3). We and others have also observed falsely increased human chorionic gonadotropin (hCG) using the Beckman Coulter Access Total hCG on the UniCel DxI (1). These false increases were identified by the finding of substantially lower results following high-speed recentrifugation and reanalysis, suggesting a particulate interference. However, the sources of these false increases were not previously determined. In reviewing clinical data for patients from whom plasma samples were found to generate falsely high hCG results on the UniCel DxI in our laboratories, we found these patients had high blood leukocyte counts. Based on this finding, we investigated the interference of leukocytes in the Beckman Coulter Access Total hCG assay. This study was approved by the Emory Institutional Review Board. First, we measured hCG in leftover plasma samples from patients with known high blood leukocyte content of 25 10/ L. The thirteen randomly selected samples (10 male, 3 female) were inverted several times in the original lithium heparin plasma separator tubes (PSTs) (Becton Dickinson) to disturb any particulates settled during storage (up to 24 h refrigerated) and then measured for hCG on the Unicel DxI. Surprisingly, eleven (11/13) of these samples had markedly increased hCG concentrations exceeding the institutional reference interval of 5 mIU/mL (range: 10 – 256 mIU/ mL, mean: 106 mIU/mL) (Table 1). However, after transferring the plasma to aliquot tubes and recentrifugation, hCG decreased by an average of 93% in the 11 previously increased samples, all to 10 mIU/mL (Table 1). Recentrifugation and testing of the original PSTs resulted in less notable reductions in the interference (average decrease of 64%). False increases were not observed in eight PSTs collected from patients with low blood leukocyte content of 1 10/ L (Table 1). After finding a high incidence of falsely increased plasma hCG in the samples tested with high blood leukocyte content, we hypothesized that addition of leukocytes in known hCG-negative plasma would lead to false hCG increases. At three separate institutions, an hCG-negative plasma sample with undetectable hCG on the Unicel DxI was identified, transferred to an aliquot tube, spiked with a leukocyte-enriched buffy coat Table 1. Plasma samples from 13 patients with high (>25 10/ L) and 8 patients with low (<1 10/ L) blood leukocyte counts were measured for hCG (Beckman Coulter Access Total hCG) on the DxI in the original lithium heparin PST and after transferring plasma to an aliquot tube and recentrifuging.