Fluctuations in hematological athlete biological passport biomarkers in relation to the menstrual cycle

The interpretation of athlete biological passport (ABP) is strengthened by understanding the natural fluctuations in its biological parameters. Here we have assessed the influence of the menstrual cycle on the hematological module of the ABP. Seventeen women with regular menses were included. Blood samples were collected once a week for two consecutive cycles and analyzed for hematological parameters. Menstrual phases were hormonally determined. The intra-individual variation in the hematological parameters was similar between the two cycles. Reticulocyte percentage was significantly lower in the follicle phase (median 0.95%) than in the ovulatory (median 1.10%) and luteal phases (median 1.16%), P = 0.006, whereas no differences were found in hemoglobin concentration, hematocrit, red blood cell count, or red blood cell indices. When the values were entered into the ABP model, findings outside the program-calculated individual thresholds were identified in two participants. One woman showed an atypical low OFF-score in the last sample collected, mainly because of increased reticulocyte percentage. This was likely a response to treated insufficient iron stores. One woman displayed an atypical hemoglobin value at the lower limit 2 weeks after ovulation, which was likely due to fluctuations in plasma volume. In conclusion, the ABP parameters in general are stable throughout the menstrual cycle. Significant differences between the menstrual phases were found in reticulocytes; however, the variation was not related to findings outside the individual thresholds, except in one individual. Moreover, our results highlight the importance of having information about iron supplementation available when evaluating hematological passports.