Purpose: This investigation quantified the effect of changes in plasma osmolality on the measurement of hematocrit (Hct) and the implications for the subsequent use of these data to calculate changes in plasma volume and application to the World Anti-Doping Agency Athlete Biological Passport.
Methods: Two groups of eight male volunteers visited the laboratory after an overnight fast. In study 1, a 20-mL blood sample was collected and aliquoted into collection tubes containing saline of varying concentrations to alter the sample osmolality. In study 2, plasma osmolality was manipulated in vivo through prolonged exercise. Samples were analyzed for hemoglobin concentration and Hct using manual methods and using an automated hematology analyzer (AHA).
Results: Changes in blood, plasma, and red cell volumes were calculated. Although AHA Hct values did not change (P = 0.652), spun packed cell volume fell progressively as the osmolality of the sample increased (P < 0.001, study 1). Consequently, there was a significant increase in apparent plasma volume as osmolality increased (P < 0.001): regression analysis revealed that a 10 mOsm·kg−1 change in plasma osmolality produced a difference of 0.8 Hct units and a 1.6% change in plasma volume. In study 2, exercise produced a 12 ± 3 mOsm·kg−1 increase in plasma osmolality. No difference in Hct was apparent at rest (P = 0.659), but spun packed cell volume was 1.0 ± 0.9 Hct units lower during exercise compared with AHA data (P < 0.001). There was a difference in the degree of plasma volume change calculated, with a reduction of 8.7% ± 3.4% and 11.3% ± 3.5% reported with the manual and AHA methods, respectively (P = 0.002).
Conclusions: Conditions or interventions that result in a marked change in plasma osmolality produce a discrepancy in Hct measured using an AHA, consequently introducing errors into any calculation of changes in plasma volume using these data. These findings may also have implications for the measurement of Hct by World Anti-Doping Agency-accredited laboratories.