Yeah if you have too many red blood cells, due to doping, your blood can thicken; increasing the strain on your heart and possibly leading to clotting and stroke.
Really they use it as a doping screening tool for borderline cases. Believe it or not the drugs test are not perfect so they use this as a simple alternative. The samples are now back tested after new techniques are developed.
Hematocrits in the 58-60 range are the upper limits of medically acceptable. Some people with borderline arteries will be symptomatic at or just below this level (claudication or other ischemic signs). The treatment is to stick a needle in, connect to a vacuum jar and suck out 500-750ml of blood. Athletes rarely get anywhere near this level with either blood doping or EPO variants. A simple screen is to draw a few ml of blood and do a hematocrit or hemoglobin level and if it goes above a certain level the athlete can be suspended or DQ. It is unusual for women to be above 45% and men above 50% (in fact most women do well to be above 40%) but the exact DQ levels escape me now. More sophisticated tests will fractionate the blood to detect donor blood from another person but don't catch blood drawn the same person and stored, to be infused shortly before an event. Blood can be stored long enough for bone marrow to replete the blood lost in the drawing. This is only good for a unit or so, not two units or more. EPO detection can be done if suspected but is a much more involved test. Upper echelon athletes in individual sports that have a lot of money involved are used to random urine and blood checks. Golfers and tennis players and team sports have escaped so far though baseball and football are being scrutinized.