There is a very common temptation among those who train: turning a blood test into a kind of “performance exam.” The more parameters, the better. And if one appears slightly outside the laboratory’s reference range, we immediately think that something is wrong. The reality is quite different.
A blood test is not a direct measure of how far you’re going to run, how much weight you’ll lift, or whether you’re recovering well from the last session. It is used to detect health problems, nutritional deficiencies, and specific alterations that can affect performance. And, above all, it allows tracking an athlete’s evolution over time.
In fact, current reviews on biomarkers in sport emphasize a fundamental idea: results must be interpreted taking into account the athlete’s usual level, recent training, nutrition, hydration, sleep, sex, recent illnesses, and the sports context. An isolated value can tell a lot less than it seems.
1. Complete blood count: the starting point
If we had to choose one essential block, it would be the complete blood count.
Here we find several parameters, but for the athlete there are four that are especially interesting:
The hemoglobin is particularly important because it is responsible for transporting oxygen from the lungs to the tissues. If anemia is present, the capacity to transport oxygen can decrease and aerobic performance can be affected.
As a general reference, the WHO considers anemia in adults as hemoglobin below 13 g/dL in men and 12 g/dL in nonpregnant women. But in athletes, you must be careful not to interpret the number in isolation: training can increase plasma volume and cause a “hemodilution” that changes some blood values without necessarily indicating disease.
Therefore, more than chasing a “perfect” hemoglobin, it’s important to know your habitual value and monitor its evolution.
2. Ferritin: probably the most important nutritional parameter
If you practice endurance sports, you’re a female athlete, have heavy menses, follow a vegetarian or vegan diet, or have experienced an unexplained drop in performance, there is one value that deserves special attention: the .
Ferritin serves as an indicator of the body’s iron stores.
And here appears one of the major confusions: you can have a normal hemoglobin and, nonetheless, have low iron stores.
In sports medicine, it is commonly considered that ferritin below 15 µg/L indicates practically exhausted stores, and that between 15 and 30 µg/L there are low reserves. For this reason, 30 µg/L is often used as a practical cut-off in athletes.
But there’s no need to obsess about achieving an extremely high ferritin. In an athlete without a deficiency, artificially increasing iron does not automatically provide more oxygen or higher performance.
If ferritin is low, it is also necessary to investigate why: insufficient diet, menstruation, blood losses, absorption problems, or high training demands can be behind it.
3. Transferrin and transferrin saturation: completing the iron photograph
When there is a suspicion of iron deficiency, ferritin alone is not always enough.
The transferrin is the protein responsible for transporting iron in the blood, and the transferrin saturation tells us how much of that transport capacity is occupied by iron.
Serum iron, by itself, is quite less useful: it can vary during the day and be affected by different factors.
Therefore, when iron deficiency is suspected, it makes more sense to assess jointly: hemoglobin + MCV + ferritin + transferrin/saturation.
This combination helps distinguish better between a simple isolated variation and a real problem with iron metabolism.
4. Vitamin B12 and folate: important, but not for everyone
Vitamin B12 and folic acid participate in red blood cell formation and nervous system function. They are especially relevant if you follow a vegan or vegetarian diet, have digestive or absorption problems, or there is an abnormal hemogram.
Vitamin B12 deserves particular attention because a deficiency can cause anemia and neurological symptoms. In athletes with a properly balanced diet there is no reason to try to elevate it indefinitely through supplements.
In other words: the goal is to correct a deficiency, not to achieve an impressive figure on the blood test.
5. Vitamin D: more related to health than to “more performance”
Vitamin D has become one of the star values in sports analytics. It makes sense to monitor it, especially in athletes with limited sun exposure, training mostly indoors, restrictive diets, or a history of bone problems.
Vitamin D participates in bone and muscle health, so a deficiency can be relevant for an athlete. However, having a higher vitamin D level than another athlete does not automatically translate into better performance. Therefore, we should not turn it into a performance marker, but fundamentally into a health indicator and a marker of nutritional sufficiency. Reviews on sports lab testing include vitamin D, calcium, and markers of bone metabolism, especially when there is a risk of stress injuries.
6. Glucose, lipid profile and kidney function: health also matters
A trained athlete remains a person who needs to monitor cardiovascular and metabolic health. Fasting glucose, total cholesterol, HDL, LDL and triglycerides can be part of a general blood test, especially if there is a family history, overweight, hypertension, or other risk factors.
It is also useful to check kidney function using parameters such as creatinine and urea, although the athlete has a peculiarity: a muscular person or someone who has trained recently can show higher creatinine without necessarily having a kidney disease.
The same goes for urea, which can rise with a high-protein intake, dehydration, or a heavy training load. Therefore, a result slightly outside the reference range should not automatically be interpreted as pathological.
7. Transaminases: beware of alarming signs from a raised AST
AST, ALT, GGT and other liver markers commonly appear in a blood test. But the athlete has an important peculiarity: some enzymes we associate with the liver are also present in muscle. After intense training, especially strength work, AST may rise and CK (creatine kinase) may also rise as a consequence of muscle stress.
This explains why a test performed immediately after a hard session can look quite different from one taken after several days of recovery. In a study of elite athletes, the most frequent alterations during routine screening were indeed increases in AST, phosphate, CK, urea, and bilirubin, and many of them were not clinically relevant.
The lesson is clear: do not interpret a sports blood test as if it were that of a sedentary person who has not trained for several days.
8. CK: does it help to know if you’re recovered?
Creatine kinase (CK) rises when there is muscle damage or stress. It can be interesting to interpret the response to a training load, especially if you have prior individual values. But it is not a universal “green light.”
A high CK does not necessarily mean you are overtrained, nor does a normal CK guarantee you are perfectly recovered. The differences between individuals are enormous, and recent exercise has a huge influence on the result. If you had a blood test 24–48 hours after running a marathon and the doctors didn’t know you had run a marathon, they would likely admit you to hospital. CK levels can soar massively after a marathon, typically reaching values around 10,000 to 20,000 U/L (units per liter), whereas a normal reference range is usually between 25 and 200 U/L.
Current recommendations on biomarkers of training load precisely warn about this difficulty: values must be interpreted relative to the individual’s level and context, not solely using population ranges.
9. C-reactive protein: is there inflammation?
CRP can be used as an inflammation marker. It can be interesting when there are symptoms, infections, injuries, or a concrete clinical suspicion. But it is not a direct marker of “recovery.” A small elevation can have multiple explanations, including a response to exercise.
That’s why it doesn’t make much sense to perform a CRP after every week of training to decide whether to rest.
10. And testosterone, cortisol and other hormones?
Here is where many sports blood tests start to become a collection of numbers hard to interpret. Testosterone, cortisol, TSH, T3, T4, and other hormones can be very useful when there is a concrete clinical question. But they should not be used as a daily marker of “anabolism,” “stress,” or “fatigue.”
Cortisol, for example, varies with the time of day, stress, sleep, and exercise. Testosterone also shows significant variability. In athletes with low energy availability there can be disturbances in different hormonal axes. In this context, the blood test can be part of a broader RED-S (relative energy deficiency in sport) assessment, but it should never be interpreted in isolation. The International Olympic Committee’s consensus on RED-S recommends integrating clinical, physiological, and performance indicators.
So, what should an athlete ask for?
For a healthy person who trains regularly, a reasonable starting point would be:
Basic
Full blood count
Iron
Biochemistry
Metabolism
According to the athlete
Add vitamin D, vitamin B12, folate, TSH, CRP, CK or other parameters when there are risk factors, symptoms, a specific diet, a high training load, or a medical indication.
How often should you get a blood test?
There is no universal frequency. A healthy athlete can have periodic checks as part of medical examinations, while someone with iron deficiency, symptoms, significant dietary changes, or a very high training load may require more frequent checks.
And there is something even more important than frequency: repeating the blood test under comparable conditions. If we want to understand a athlete’s evolution, it is much more useful to compare results obtained in similar circumstances than to obsess over the laboratory’s reference range.