What it measures.
Hematocrit measures what percentage of your blood volume is occupied by red blood cells. While hemoglobin tells you oxygen-carrying capacity, hematocrit reveals blood 'thickness.' Both very low (anemia) and very high (polycythemia) hematocrit have clinical significance. In endurance athletes and testosterone users, elevated hematocrit is common and important to monitor.
The percentage of blood volume comprised of red blood cells, expressed as a decimal (0.45) or percentage (45%). Determined by centrifuging blood to separate cells from plasma and measuring the packed red cell layer.
Why it matters.
Hematocrit affects blood viscosity and oxygen delivery. Too low = anemia with fatigue and poor exercise capacity. Too high = thick blood that strains the heart and increases stroke/clot risk. Optimal hematocrit balances oxygen delivery with flow characteristics.
Physiology.
Hematocrit is determined by the balance between red blood cell production (in bone marrow, stimulated by erythropoietin from kidneys) and destruction (in spleen). Dehydration concentrates blood, falsely raising hematocrit. Overhydration dilutes it. Altitude, testosterone, and EPO increase true RBC production.
Testing & preparation.
How to prepare
- Stay normally hydrated (dehydration falsely elevates)
- Avoid testing after major fluid shifts (intense exercise, illness)
- Fasting not required
When to test
Part of standard CBC; specifically monitor if on testosterone therapy, living at altitude, or with symptoms of polycythemia
How often
Annually for general health; every 3-6 months if on testosterone replacement
Interpretation.
High hematocrit
Common causes:
- Dehydration (most common cause of spuriously high Hct)
- Testosterone replacement therapy
- Polycythemia vera (bone marrow disorder)
- Sleep apnea (chronic hypoxia stimulates EPO)
- Chronic lung disease (COPD, pulmonary fibrosis)
- High altitude living
- EPO doping in athletes
- Kidney tumors producing EPO
Implications:
- Increased blood viscosity and clot risk
- Higher blood pressure and cardiac strain
- Above 52%: significant stroke/DVT risk
- May indicate underlying hypoxic condition
- TRT patients: may need phlebotomy or dose reduction
Low hematocrit
Common causes:
- Iron deficiency anemia
- Vitamin B12 or folate deficiency
- Chronic disease (inflammation, kidney disease, cancer)
- Blood loss (GI bleeding, heavy menstruation)
- Hemolysis (RBC destruction)
- Bone marrow disorders
- Overhydration (dilutional)
Implications:
- Reduced oxygen delivery to tissues
- Fatigue, weakness, shortness of breath
- Poor exercise tolerance
- Need to determine cause (production vs. destruction vs. loss)
Optimization.
Diet
- If low: iron-rich foods, B12, folate (address deficiency cause)
- If high: stay well hydrated, limit iron supplementation
- Adequate protein for RBC production
- Beet juice may modestly help athletic performance
Lifestyle
- Address sleep apnea (high hematocrit is a red flag)
- Blood donation if naturally high (benefits others, reduces Hct)
- For TRT users: regular phlebotomy as needed
- Altitude training increases Hct (athletic advantage)
Supplements
- Iron, B12, folate only if documented deficiency
- EPO-boosting supplements (often ineffective and risky)
FAQs.
I'm on testosterone—how often should I check hematocrit?
Every 3-6 months, especially in the first year. TRT commonly raises hematocrit because testosterone stimulates erythropoietin. If Hct exceeds 50-52%, you may need therapeutic phlebotomy (blood donation equivalent) or TRT dose reduction. Untreated high Hct on TRT increases clot and cardiovascular risk.
My hematocrit was 54%—should I be worried?
Yes, 54% is significantly elevated and increases your risk of blood clots, stroke, and cardiovascular events. First, rule out dehydration (re-test when well hydrated). If genuinely elevated, investigate causes (sleep apnea, TRT, polycythemia vera). You likely need therapeutic phlebotomy to bring it below 50%.
Is higher hematocrit better for athletic performance?
Up to a point. Higher Hct means more oxygen-carrying capacity—that's why altitude training and EPO doping work. But beyond ~50-52%, blood becomes too viscous, impairing flow and increasing heart strain. The performance sweet spot is probably 45-50% for most people, though this is individual.