Alterations in hematologic indices during long-duration spaceflight

Background: Although a state of anemia is perceived to be associated with spaceflight, to date a peripheral blood hematologic assessment of red blood cell (RBC) indices has not been performed during long-duration space missions. Methods: This investigation collected whole blood samples from astronauts participating in up to 6-months orbital spaceflight, and returned those samples (ambient storage) to Earth for analysis. As samples were always collected near undock of a returning vehicle, the delay from collection to analysis never exceeded 48h. As a subset of a larger immunologic investigation, a complete blood count was performed. A parallel stability study of the effect of a 48h delay on these parameters assisted interpretation of the in-flight data. Results: We report that the RBC and hemoglobin were significantly elevated during flight, both parameters deemed stable through the delay of sample return. Although the stability data showed hematocrit to be mildly elevated at +48h, there was an in-flight increase in hematocrit that was ∼3-fold higher in magnitude than the anticipated increase due to the delay in processing. Conclusions: While susceptible to the possible influence of dehydration or plasma volume alterations, these results suggest astronauts do not develop persistent anemia during spaceflight.