Key takeaways
Iron status is the most useful athlete blood test, as iron deficiency is found in roughly 15 to 35% of female and 5 to 11% of male athlete groups
A slightly low haemoglobin in an endurance athlete can be sports anaemia, caused by a larger plasma volume rather than a lack of red cells
Test rested, in the morning and under the same conditions each time, and compare results with your own previous values and with your doctor
Training data from your watch shows what you did. Blood work shows some of what your body needs to keep doing it. For endurance athletes, a handful of markers can explain unexpected fatigue, a stalled VO2max or frequent illness. Here is what to test, how to read it and when to have blood drawn.
Why athletes test differently
A standard check-up compares you with a general population. Athletes have some quirks: bigger plasma volume, more muscle damage, higher iron losses and different "normal" values for some enzymes. A 2017 review in the Journal of Strength and Conditioning Research (Lee et al.) suggested that athlete panels cover nutrition and metabolic health, hydration, muscle status, endurance performance, injury risk and inflammation. Its main practical point: repeated measurements let you build a personal baseline, which is often more useful than a single result checked against a lab range.
The core panel
| Marker | Why it matters for endurance | What to look out for |
|---|---|---|
| Full blood count (haemoglobin, haematocrit, MCV) | Haemoglobin carries oxygen, a key limit on VO2max | Low values, small red cells (low MCV) |
| Ferritin and iron studies | Iron stores for red cells and muscle enzymes | Low ferritin, low transferrin saturation |
| CRP | Inflammation; also helps interpret ferritin | Raised CRP can mask low iron stores |
| Vitamin D (25-OH-D) | Bone, muscle and immune function | Very low or very high values |
| B12 and folate | Needed to make red blood cells | Low values, large red cells (high MCV) |
| Thyroid (TSH, sometimes free T4) | Energy, heart rate, body weight | Unexplained fatigue, weight change |
| Lipids or ApoB | Long-term heart health, even in fit people | High LDL or ApoB |
| HbA1c | Average blood sugar over about three months | Rising values over time |
Iron: the most useful athlete test
Iron deficiency is common in sport. A 2019 review by Sim and colleagues reported deficiency in roughly 15–35% of female and 5–11% of male athlete groups. Training raises hepcidin, the hormone that controls iron absorption, for a few hours afterwards, and heavy sweating, foot-strike haemolysis, menstruation and low energy intake add to losses. Altitude camps increase iron demand further (see altitude training).
Ferritin reflects iron stores, but it also rises with inflammation. A Swiss review (Clénin, 2017) recommended checking CRP alongside ferritin to avoid false reassurance and suggested a ferritin cut-off of 30 µg/L for diagnosing iron deficiency in healthy people over 15. Sports bodies sometimes use other thresholds, so ask your doctor which applies. The same review warns that taking iron "just in case" when stores are normal is ineffective and may be harmful. Read more on our iron supplement page.
Haemoglobin and "sports anaemia"
Endurance training expands plasma volume. As Mairbäurl (2013) explains, this can push haematocrit and haemoglobin concentration slightly down even though trained athletes actually have more total red cell mass than sedentary people. This "sports anaemia" or dilutional pseudoanaemia is not anaemia in the clinical sense. The catch: it looks similar to early iron deficiency on a single test, so a low result should be checked with ferritin, MCV and your history rather than dismissed.
Vitamin D
Vitamin D supplements are popular with athletes, but more is not always better. A 2018 review (Owens et al.) noted that data have consistently failed to show a link between blood vitamin D and bone health in athletes, raised concerns about very high doses, and pointed out that Black athletes often have low measured levels without obvious harm. Testing makes most sense if you train indoors, live far from the equator, cover your skin or have had stress fractures. See our vitamin D page.
Muscle enzymes (optional)
Creatine kinase (CK) is often high in athletes. Normal CK varies with age, sex, ethnicity, muscle mass and activity (Brancaccio et al., 2008), so a raised value after training is expected. A high CK after a proper rest is different and should be investigated.
When to have blood taken
- Same conditions every time: morning, fasted if lipids or glucose are included, well hydrated.
- After a rest or easy day. Hard or very long sessions can temporarily raise CK, CRP and other markers, so a common approach is to wait a day or two after your last big session. Avoid testing just after a race, a long flight or an illness.
- Women: note your cycle day, as menstruation can affect iron markers (see the menstrual cycle and VO2max).
How often?
| Situation | Suggested frequency |
|---|---|
| Healthy recreational athlete | Once a year |
| High training volume, female, vegetarian or vegan | Every 6 months, or as advised |
| Altitude camp | Before, and possibly after |
| Unexplained fatigue or performance drop | As soon as practical |
| On iron treatment | As your doctor schedules |
Making sense of the results
Look at trends, not single numbers. Pair blood work with your training data: a falling VO2max estimate, a resting heart rate that creeps up (see resting heart rate and HRV) or unusual breathlessness are good reasons to test. Fatigue is often about training load, sleep or low energy intake rather than a lab abnormality. Blood work can't tell you everything, but it can rule out the common, fixable problems.
If you're interested in broader health markers, see which longevity biomarkers matter and our healthspan checklist. If you take supplements, it's also worth knowing how products are tested for contaminants. To track your fitness alongside your blood results, start with our VO2max calculator.
Always discuss your results with a doctor before starting supplements or changing treatment. Lab ranges vary, and some findings need follow-up that no article can replace.
Frequently asked questions
What blood tests should an athlete get?
A sensible core panel is a full blood count, ferritin with iron studies and CRP, vitamin D, B12 and folate, thyroid function, lipids and HbA1c. Your doctor may add others based on symptoms, diet and history.
What is athlete's anaemia?
Endurance training expands plasma volume more than red cell mass, so haemoglobin concentration can read slightly low even though total red cell mass is higher than in untrained people. It is not true anaemia, but a doctor should rule out iron deficiency first.
How long after training should I have blood taken?
A common practical approach is to test in the morning after a rest or easy day, ideally a day or two after your last hard or very long session, because hard exercise can temporarily raise muscle enzymes and inflammation markers.
How often should athletes get blood work?
Many athletes test once or twice a year, with extra tests during heavy training blocks, altitude camps, after illness or when performance drops without explanation. Discuss the right schedule with your doctor.
Sources
- Lee EC, Fragala MS, Kavouras SA, et al. Biomarkers in sports and exercise: tracking health, performance, and recovery in athletes. J Strength Cond Res, 2017;31(10):2920-2937.
- Sim M, Garvican-Lewis LA, Cox GR, et al. Iron considerations for the athlete: a narrative review. Eur J Appl Physiol, 2019;119(7):1463-1478.
- Clénin GE. The treatment of iron deficiency without anaemia (in otherwise healthy persons). Swiss Med Wkly, 2017;147:w14434.
- Mairbäurl H. Red blood cells in sports: effects of exercise and training on oxygen supply by red blood cells. Front Physiol, 2013;4:332.
- Owens DJ, Allison R, Close GL. Vitamin D and the athlete: current perspectives and new challenges. Sports Med, 2018;48(Suppl 1):3-16.
- Brancaccio P, Maffulli N, Buonauro R, Limongelli FM. Serum enzyme monitoring in sports medicine. Clin Sports Med, 2008;27(1):1-18.