Red blood cell morphology after a 100-km run

We studied the red blood cells of male athletes competing in a 100-km run. The RBC, haematocrit, and red cell indices were unaffected. The red cell morphology was studied in a wet preparation with light microscopy. A slight stomatocytosis was observed after the run compared to a control day (12.4 +/- 1.8% vs 4.3 +/- 1.8%, P less than 0.05). This difference persisted when runner red cells were incubated in buffer. Red cells from a normal donor did not undergo stomatocytic transformation when incubated in runner plasma. These data indicate that stomatocytosis after long-distance running is not caused by a plasmatic factor but is a change in the red cell membrane itself.