Hematocrit: What the Test Shows and Why It Matters for Athletes
Hematocrit is one of the simplest numbers on a complete blood count, yet it says a lot about an athlete's oxygen-carrying capacity, hydration and health risks. Too low, and performance suffers; too high, and the blood becomes thick enough to raise the risk of clots. Here is what the test shows and how to interpret it.
What hematocrit measures
Hematocrit (Hct) is the percentage of blood volume occupied by red blood cells. It is reported as part of a complete blood count (CBC), along with hemoglobin and red cell indices.
Typical adult reference ranges in US labs are roughly 41–50% for men and 36–44% for women, though exact values vary by laboratory and altitude.
Red blood cells carry oxygen bound to hemoglobin, so hematocrit is closely linked to aerobic capacity. Mairbäurl (2013) reviewed how training influences red cell mass and oxygen delivery.
Hematocrit depends on both red cell mass and plasma volume. Changes in either can shift the number.
Why athletes' results can be misleading
Endurance training expands plasma volume more than red cell mass, which can lower hematocrit slightly. This "sports anemia" or dilutional pseudoanemia is usually harmless (Sawka et al., 2000; Eichner, 1992).
Dehydration does the opposite: losing plasma concentrates red cells and raises hematocrit temporarily.
Altitude stimulates erythropoietin (EPO) and increases red cell mass over weeks.
True anemia, often from iron deficiency, is common in female athletes and endurance athletes. Peeling et al. (2008) described how exercise-induced inflammation and hepcidin can impair iron absorption.
High hematocrit and testosterone
Testosterone stimulates red blood cell production. Erythrocytosis is the most common laboratory side effect of testosterone therapy (Ohlander et al., 2018).
The Endocrine Society guideline (Bhasin et al., 2018) advises against starting testosterone therapy when hematocrit is above 50% and recommends stopping or adjusting therapy if it rises above 54%.
High hematocrit increases blood viscosity, which may raise the risk of thrombosis, stroke and heart attack, though the exact risk level is debated.
Non-medical use of anabolic steroids, EPO or other substances can push hematocrit to dangerous levels.
| Hematocrit range (men) | Typical interpretation |
|---|---|
| Below ~41% | Possible anemia or dilution; check hemoglobin, iron studies |
| ~41–50% | Typical reference range |
| 50–54% | Elevated; investigate causes, check hydration |
| Above 54% | Clinically significant erythrocytosis; needs medical evaluation |
Hematocrit in anti-doping
Before reliable EPO tests existed, some sports federations used hematocrit thresholds as a health and anti-doping measure.
Today, the Athlete Biological Passport (Sottas et al., 2011) tracks hemoglobin, reticulocytes and derived markers over time to detect blood manipulation.
Unexplained changes can lead to investigation even without a positive test.
Athletes should keep records of altitude training, illness and hydration status that could explain fluctuations.
How to get an accurate result
- Be well hydrated, but avoid excessive water intake right before the test;
- Avoid intense exercise for 24 hours before testing;
- Test at a similar time of day for comparisons;
- Inform your doctor of altitude stays, blood donations, supplements and medications;
- Interpret hematocrit together with hemoglobin, red cell count and iron markers.
Ferritin and transferrin saturation help identify iron deficiency even before anemia develops.
A single high result in a dehydrated athlete should be repeated under standard conditions.
Persistently elevated hematocrit requires medical evaluation for causes such as sleep apnea, smoking, lung disease or polycythemia vera.
Conclusions from the editorial team
Hematocrit reflects red cell mass and plasma volume, making it useful for assessing oxygen-carrying capacity, hydration and health risks.
Endurance training can lower it slightly, dehydration and altitude can raise it, and testosterone or EPO use can push it to dangerous levels.
Values above 54% in men require medical attention, while low values warrant iron and anemia evaluation.
Related topics on our site: how to prepare for an SHBG test, testicular atrophy, and anti-doping rules in Germany.
References
- Bhasin S, Brito JP, Cunningham GR, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2018;103(5):1715–1744.
- Sawka MN, Convertino VA, Eichner ER, Schnieder SM, Young AJ. Blood volume: importance and adaptations to exercise training, environmental stresses, and trauma/sickness. Med Sci Sports Exerc. 2000;32(2):332–348.
- Mairbäurl H. Red blood cells in sports: effects of exercise and training on oxygen supply by red blood cells. Front Physiol. 2013;4:332.
- Ohlander SJ, Varghese B, Pastuszak AW. Erythrocytosis following testosterone therapy. Sex Med Rev. 2018;6(1):77–85.
- Sottas PE, Robinson N, Rabin O, Saugy M. The athlete biological passport. Clin Chem. 2011;57(7):969–976.
- Peeling P, Dawson B, Goodman C, Landers G, Trinder D. Athletic induced iron deficiency: new insights into the role of inflammation, cytokines and hormones. Eur J Appl Physiol. 2008;103(4):381–391.
- Eichner ER. Sports anemia, iron supplements, and blood doping. Med Sci Sports Exerc. 1992;24(9 Suppl):S315–S318.
