Blood tests for athletes: what to test and how to read it after training

The blood tests most worth having as an athlete are ferritin with transferrin saturation, a CBC, vitamin D, CK, a thyroid panel with free T3, morning testosterone with SHBG and hs-CRP. Hard training moves several of them. A heavy leg day can push creatine kinase into the thousands and liver enzymes above their range for a week, and a big endurance block can dilute hemoglobin. Lab ranges are built from people who did neither, so read each result against your training week and your own earlier results.

The athlete panel at a glance

A standard annual panel covers most of what a sports performance blood test sells. Several of those results move with training, some for a day and some for a week or more. The table lists the markers worth checking and what a hard week does to each one.

Iron and ferritin: the one most athletes should check first

Iron deficiency without anemia (low iron stores while hemoglobin still reads normal) is reported in roughly 15 to 35% of female athletes and 5 to 11% of male athletes ¹. Athletes lose iron through sweat, small gut bleeds during long efforts, red cells broken up by repeated foot strikes, and menstruation. Hard exercise also raises hepcidin, the hormone that blocks iron absorption from the gut, with a peak about 3 to 6 hours afterwards, so iron eaten in that window is absorbed less well ¹.

Training explains a lot of athlete iron loss, but not all of it. Iron deficiency in a man or a postmenopausal woman, or one that does not fit your training and diet, needs a doctor to look for the cause, such as bleeding from the gut or coeliac disease, before it is put down to training.

Sports scientists stage athlete iron deficiency by ferritin, the protein that stores iron and the best single read of your reserve ¹. Ferritin under 35 µg/L means stores are depleted. Ferritin under 20 µg/L with transferrin saturation (the share of your iron-carrying protein that is actually carrying iron) under 16% means iron deficiency without anemia: the body is short of iron for new red cells, but hemoglobin has not fallen yet. The third stage is iron-deficiency anemia, when hemoglobin drops below its range. A normal hemoglobin rules out only that third stage.

Ferritin also rises with inflammation, so a value drawn the day after a race or during a cold can look healthier than your stores are ¹. Check CRP on the same sample: if CRP is raised, the ferritin is probably overstating your iron. Iron studies explained covers the full panel, ferritin vs iron covers why serum iron alone is too jumpy to rely on, and iron-rich foods covers the food side.

Endurance athletes can also show a slightly low hemoglobin with healthy iron. Training expands plasma (the watery part of your blood) faster than it adds red cells, so the same red cells sit in more fluid and the concentration reads lower ¹⁰. This dilutional pseudoanemia comes with a normal ferritin and normal red cell indices. A low hemoglobin with low ferritin is different and points to iron deficiency. A low hemoglobin on its own is covered in anemia, and the CBC explained walks through the rest of the count.

CK and the muscle markers: why a hard week looks like an injury on paper

Creatine kinase (CK, an enzyme that leaks out of muscle fibres when they are damaged) usually peaks 24 to 96 hours after strenuous or unaccustomed exercise and can stay raised for several days ². Eccentric work (downhill running, the lowering phase of a lift) raises CK the most. Mougios proposed athlete reference intervals for CK that reach about 1,083 U/L in men and 513 U/L in women, several times the upper limit printed on most general lab reports ³.

Muscle also contains AST and ALT, the two enzymes most people read as liver tests. In a study of healthy men who were not used to weight training, one session of heavy lifting raised AST, ALT, LDH (another enzyme released by damaged cells), CK and myoglobin (the oxygen-holding protein in muscle), and the enzymes stayed raised for at least 7 days ⁴. The paper's title states the finding.

Muscular exercise can cause highly pathological liver function tests in healthy men.Pettersson J et al., Br J Clin Pharmacol 2008 ⁴

When AST is higher than ALT and CK is also raised, muscle is the likelier source. Liver enzymes explained and high AST cover how to tell muscle from liver when the pattern is less clear.

Hormones: testosterone, cortisol and thyroid

The Endocrine Society's 2018 guideline says testosterone should be measured in a morning fasting sample, and a low result confirmed with a repeat test before anyone draws a conclusion ⁷. Read total testosterone with SHBG (the binding protein that holds most of it) because a high SHBG, common in lean endurance athletes, can leave less free hormone even when the total looks fine. The free testosterone calculator turns total testosterone and SHBG into a free value. DHEA-S and IGF-1 are sometimes added to athlete hormone panels, but they rarely change what you do; the testosterone, SHBG and free T3 read comes first.

Eating too little for the training you do is called low energy availability, part of REDs (relative energy deficiency in sport) ⁶. It is a common reason training lowers testosterone in men, and very high endurance volume often comes with it. A low result that does not recover with rest and enough food is covered in low testosterone, the male hormone panel guide covers the full read, and the testosterone goal page covers what changes it.

Cortisol swings widely across the day and climbs after hard sessions, so one cortisol value tells you almost nothing about recovery. The thyroid panel is more useful, and how to read your thyroid panel explains the three numbers. If free T3 and TSH both look off, hypothyroidism covers the thyroid causes. For women, missed or irregular periods are the clearest REDs signal, and estradiol and the rest of the female hormone panel help put a number to it. Periods that stop for three months or more, or have not started by 15, need a doctor's assessment: REDs is one cause, and pregnancy, thyroid and pituitary causes need ruling out.

Vitamin D, B12, fuel and the minerals

Vitamin D inadequacy is common in athletes: in a meta-analysis, about 56% had a 25-OH vitamin D below 32 ng/mL, with indoor sports, northern latitudes and winter raising the risk ¹³. US labs report it in ng/mL and most other countries in nmol/L, and the vitamin D units converter switches between them. Vitamin D deficiency covers the cutoffs. Plant-based athletes should add B12 and folate, since B12 comes almost only from animal foods and supplements. Serum magnesium stays normal until total body magnesium is quite low, because the body holds the blood level steady by pulling from bone and cells, so a normal result does not prove your stores are full.

Fasting glucose and HbA1c sit in the standard panel; a low fasting glucose alongside fatigue can be another sign of under-fuelling, and is worth raising with your doctor. The omega-3 index is an optional add-on that tracks fish or algae oil intake and is not needed to read the core panel. Sodium usually reads normal, but a low sodium after a long event usually comes from drinking too much water, and confusion, headache or vomiting after a race needs urgent care.

Supplements linked to these results

Strongest evidenceIronRaises ferritin and hemoglobin when a doctor has confirmed iron deficiency; does nothing useful when stores are full, and too much iron is harmful, so test first and agree a dose with your doctorIn our shop

Strong for deficiencyVitamin DCorrects a low 25-OH D; performance gains beyond correcting deficiency are inconsistentIn our shop

Mixed evidenceMagnesiumHelps when intake or levels are low; little effect on performance in people who are repleteIn our shop

Strongest evidenceCreatineImproves repeated high-intensity work; raises serum creatinine, with no evidence of kidney harm in people with healthy kidneys. Anyone with kidney disease should ask their doctor firstIn our shop

Inflammation and immune markers

CRP rises sharply in the day or two after a marathon or ultra, while regular training is linked to a lower resting CRP ¹¹. An hs-CRP drawn two days after a race reads the race. Inflammation markers explained and hs-CRP vs CRP cover which test to read when. White cell counts in endurance athletes often sit at or slightly below the bottom of the range and stay there year after year. A stable low WBC with normal neutrophils (the white cells that fight bacterial infection) and no infections is usually that athlete's baseline, which your doctor can confirm. A low neutrophil count, a count that keeps falling, or frequent or severe infections need a doctor to look at it; low WBC covers when a low count needs a closer look.

Why out of range can be normal for you

Lab reference ranges are built mostly from untrained adults, and reference ranges explained covers how they are set. For an athlete, five results commonly flag without meaning disease: high CK, high AST or ALT after lifting, slightly low hemoglobin from plasma expansion, low WBC, and high creatinine. A training effect should settle on a repeat test after rest; if it does not, or you have symptoms, bring both results to your doctor.

Creatinine comes from muscle, so a muscular person, someone taking creatine ⁹, or someone on a high-protein diet can show a raised creatinine, a higher urea and a falsely low eGFR (estimated glomerular filtration rate, the lab's estimate of kidney filtering, calculated from creatinine). KDIGO, the kidney guideline group, recommends cystatin C to confirm eGFR when creatinine is unreliable, for example because of high muscle mass ⁸. Cystatin C is made by every cell at a steady rate, so muscle mass and creatine barely move it; cystatin C vs creatinine covers the two side by side.

CalculatoreGFR calculatorRun your creatinine through eGFR, then compare it with a cystatin C result if your creatinine is flagged high.Work it out

High-sensitivity troponin, a heart muscle marker, can also rise in healthy people after a marathon or other long event and usually settles within 24 to 72 hours ¹². Chest pain, fainting or breathlessness makes any raised troponin a medical result, not a training effect.

The useful comparison for an athlete is with your own earlier results drawn under the same conditions. Trends beat single readings explains why a change from your baseline tells you more than a flag.

How to prepare so your results mean something

1. 1Rest 48 to 72 hoursKeep the two or three days before the draw easy, with no unfamiliar or eccentric work. If liver enzymes matter, leave a week after heavy lifting.
2. 2Morning, fasted, hydratedDraw before 10 am after an overnight fast, with water as normal. Testosterone and cortisol depend on the hour.
3. 3Not ill or freshly back from altitudeA cold raises CRP and ferritin; altitude shifts hemoglobin. Postpone the test if either applies.
4. 4Same conditions every timeSame lab, same time of day, same gap after training. Women can note cycle day.
5. 5Log your trainingWrite down the last two weeks of load beside the results, so a change can be traced to the training or ruled out.

How to prepare for a blood test covers fasting and medicines in general, and alcohol and blood tests covers what a weekend does to liver numbers.

Can a blood test tell you you're overtrained?

No single blood marker diagnoses overtraining syndrome. The joint consensus of the European College of Sport Science and the American College of Sports Medicine treats it as a diagnosis of exclusion, made after ruling out illness, iron deficiency, endocrine disease and under-eating ⁵. Blood tests find or rule out those other causes. The consensus states the training principle directly.

Successful training must involve overload but also must avoid the combination of excessive overload plus inadequate recovery.Meeusen R et al., ECSS/ACSM joint consensus, Med Sci Sports Exerc 2013 ⁵

Several findings together make overtraining more likely: a falling testosterone to cortisol ratio across several tests, CK that stays high after a rest week, falling ferritin and free T3, with stalled performance, poor sleep and low mood. Overtraining syndrome covers the full picture, and fatigue and chronic fatigue syndrome cover tiredness that does not lift with rest.

What to test by sport and by person

How often to test

• A baseline in an easy week, when you feel well.
• Before the season or a major training block.
• After a heavy block or a run of poor sessions.
• When symptoms appear: unusual fatigue, breathlessness on easy runs, missed periods, frequent illness.
• Ferritin 8 to 12 weeks after a doctor starts you on iron, to see whether it is working.

The body performance goal page gathers the markers, guides and supplements for training in one place.

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