Comment
Currently, no freediving organization does blood testing. So your favourite star freediver could be blood doping or using EPO and no one would know.
In cycling, a blood test with hematocrit of more than 50% does not mean your career is over. In fact, it doesn't prove anything. For them, it is simply not safe for you to race in that state. The star of cycling recently measured at 52%, which to most people meant he 'doped', when in fact it is possible to reach more than 50% naturally. Excluding the athlete from competing is not an accusation of doping, because neither EPO nor blood doping can yet be conclusively proven. They simply prevent the athlete from competing due to safety reasons.
Keep in mind that enormously thick blood is possible naturally. Andean natives (living at 5000m+) frequently have hematocrits of 75-80%, but their blood does not get excessively thick, because their body reduces their fibrinogen and other viscosity agents to partially compensate for the increased viscosity due to the extra hemoglobin. The problem is if you put your hematocrit way up there with EPO or blood doping (i.e. artificially & suddenly), then your body was not responsible for the increase, and your body is not able to adjust other viscosity agents to compensate, so you can suffer a stroke/heart attack.
So, high hematocrits can be very safe, but only if your body reached that level naturally.
Eric Fattah
BC, Canada