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How does that help? I think it's not a simple measure of hematocrit for the passport, they're measuring reticulocytes (young RBCs) and stuff that indicates excess newly formed RBC compared to normal as well as the chemicals that prompt the body to make them and the ratio between new and old RBCs so if a rider has all new RBCs it's going to look well dodgy, regardless of altitude.
https://www.wada-ama.org/en/questions-answers/athlete-biological-passport#item-436
Or use it when at altitude?