Hematocrit is the ratio of red blood cells to total volume of your blood. A normal ratio is around 45 for men and 40 for women. It's very important to keep this ratio bc too many RBCs can cause your blood to become more viscous, sludgy and harder to pump. If you workout, and if you take any type of exogenous hormones/androgens, either of those things can cause your body to increase its total RBC count, and by extension increase your hematocrit ratio.
Especially if you take certain steroids like anadrol and equipoise,as these were designed specifically to increase RBC(anadrol was originally for anemia). Donating blood every 8 weeks is a great way to not only thin out your blood and reduce your hematocrit ratio, but also potentially save lives. I strongly suggest if you are a bodybuilder/powerlifter/competitor or gym rat who takes anything that resembles androgens, including prohormones and precursors, to get your hematocrit levels checked and donate blood every 8 weeks. Your health should be always be your first concern.