Control of analyzer slope and intercept in the measurement of packed red cell volume (PCV): part II

A majority of packed cell volume (PCV) assays performed in the United States are performed by multichannel hematology analyzers rather than by centrifugation of blood samples in glass capillary tubes. PCV results from both analytical approaches were compared to the recently described ICSH reference method to determine the effect of assay slope on clinical utility. Though not controllable by the end user, the slope of the PCV assay versus the reference was satisfactory when performed in centrifuged glass capillaries. In the small sample of multichannel analyzers surveyed, only two of six machines functioned as well in this respect as did the glass capillaries. The worst of the unsatisfactory multichannel analyzers would have identified correctly only about half of the anemic and the polycythemic patients in the 12,623 males surveyed in the Third National Health and Nutrition Examination Survey (NHANES III). The performance on the females in this U.S. Department of Health and Human Services study would have been similarly poor. It is important that manufacturers of multichannel analyzers attend to the slopes of their PCV assays if the results are to be diagnostically useful.