Blood Tests for Athletes: The Markers That Actually Matter
A cyclist sent me a 42-marker panel last month and asked which number explained why his legs felt heavy on climbs. Thirty-eight of those markers were never going to answer that question. Two of them might have. He'd paid for the wrong test and then read it in the wrong order.
The blood markers that matter most for athletes are ferritin (iron stores), vitamin D, B12 (especially for vegetarians), and RBC magnesium — not serum magnesium, which misses intracellular depletion. Test with a reason, not on a schedule, and address deficiencies through food first. Most athlete panels include markers that don't change decisions.
Two things go wrong with blood work in amateur sport. The first is testing nothing until something breaks. The second is testing everything, quarterly, and treating each out-of-range flag as a problem to fix. Both come from the same place: not knowing which numbers actually change what you do on Tuesday.
Quick Overview
Why blood tests for athletes go wrong
Reference ranges on a lab report are built from a general population, not from people doing 12 hours a week of training. A ferritin of 22 ng/mL gets flagged as normal on most Indian, UK and Australian lab reports. For a female marathon runner, that number is a reasonable explanation for why her intervals have gone backwards.
The other half of the problem is timing. A panel drawn 18 hours after a hard session reads differently from one drawn on a rest day, because inflammation lifts ferritin and hard training shifts several markers temporarily. If you're going to test, test rested, hydrated, fasted where the panel requires it, and ideally at the same point in your week each time so the numbers are comparable.
Blood work also isn't self-serve. Reading a panel and deciding what to do about it is a clinician's job, and everything below is written to help you understand what your doctor is looking at, not to replace them.
Ferritin: the marker most worth watching
Ferritin measures stored iron, and it's the marker that most often explains unexplained fatigue in endurance athletes. Low iron reduces oxygen-carrying capacity and aerobic performance well before it shows up as anaemia on a haemoglobin reading, which is why haemoglobin alone is a poor screen for athletes (Solberg & Reikvam, Life, 2023).
The practical number: most sports medicine guidance treats ferritin below 30–35 ng/mL as the point where an endurance athlete is likely to be symptomatic, and many clinicians working with runners prefer 40–50 ng/mL as the floor. Compare that to the roughly 15 ng/mL cutoff most labs use for general-population deficiency and you can see how an athlete gets told they're fine while feeling anything but.
Female athletes and vegetarian athletes need it monitored more closely. Menstrual losses, plant-source iron that absorbs poorly, and the foot-strike haemolysis and sweat losses that come with high running volume all pull in the same direction. If you're vegetarian and running 60km (about 37 miles) a week in Chennai or Chicago, this is the first marker to look at, and the iron post covers what to eat and how to pair it with vitamin C.
Test if you're fatigued without explanation, if you're a female endurance athlete, if you're vegetarian or vegan, or if you've had a sudden drop in training tolerance. Don't test it monthly. Iron stores move slowly and you'll only frustrate yourself.
Does vitamin D affect athletic performance?
Vitamin D matters for bone health and muscle function, and inadequacy is common enough in athletes to be worth checking once rather than assuming. A meta-analysis of 2,313 athletes found 56% had inadequate status, with risk rising in winter and spring and in indoor sports (Farrokhyar et al., Sports Med, 2015).
Indoor athletes carry most of the risk, which is counterintuitive if you assume sunlight is abundant where you live. A badminton player in Hyderabad, a swimmer in Manchester and a basketball player in Toronto are all training under a roof. Air quality plays in too: athletes in high-particulate cities get less usable UVB than latitude alone predicts, which is why Delhi and Beijing athletes can be low in a way the map doesn't suggest.
Most guidance treats 25(OH)D below 30 ng/mL (75 nmol/L) as insufficient. Test once, in late winter if you're in a seasonal climate, since that's the annual low point.
B12, and why vegetarian athletes need it checked
B12 comes almost entirely from animal foods, so vegetarian and vegan athletes are the group that needs it checked. In a study of young, healthy, unsupplemented Indian vegetarian graduates, half were B12 deficient and most showed raised homocysteine (Naik et al., Br J Nutr, 2018).
That figure isn't unique to India, though the prevalence there is higher because lifelong vegetarianism is more common. A vegan triathlete in Melbourne is running the same biochemical risk, just against a lower background rate.
Total serum B12 alone is a mediocre test. If it comes back borderline and symptoms suggest deficiency, holotranscobalamin or homocysteine gives a clearer picture, and it's reasonable to ask your doctor whether either is warranted. Fortified foods and dairy help, but a long-term vegan athlete generally needs supplementation rather than dietary adjustment.
Why RBC magnesium and not serum magnesium
Serum magnesium is the test almost every panel includes and almost the only magnesium test that tells you nothing useful. Roughly 1% of body magnesium sits in serum, and the body holds that fraction steady by pulling from bone and tissue, so serum can read normal while tissue stores are substantially depleted (Costello & Nielsen, Curr Opin Clin Nutr Metab Care, 2017).
Red blood cell magnesium measures the intracellular pool and reflects intake across the 120-day lifespan of the cell, which makes it the more informative request. No magnesium biomarker is perfect, and the same review is clear about that, but RBC beats serum for the question an athlete is actually asking.
Ask for it by name. Labs in India, the UK and the US will run it, but not by default, and a doctor looking at a normal serum magnesium has no reason to look further unless you raise it. The magnesium post goes deeper into cramping claims and what the evidence supports.
Should athletes worry about HbA1c?
HbA1c is unreliable in athletes in both directions, and in a healthy trained athlete it rarely changes anything. The test measures glucose bound to haemoglobin over the life of the red cell, so anything that alters red cell lifespan distorts the result independently of actual blood sugar (Radin, J Gen Intern Med, 2014). Iron deficiency lengthens red cell survival and pushes HbA1c up. Heavy endurance training shortens it and pulls HbA1c down.
That produces two failure modes I see regularly. An iron-deficient runner gets an HbA1c of 5.9% and is told she's pre-diabetic, when the number is an artefact of the same low ferritin that's flattening her training. A masters cyclist with a 4.9% reads it as proof of metabolic health when his red cells simply aren't living long enough to accumulate glucose.
There's a genuine reason to run it anyway if you're South Asian, where type 2 diabetes appears at lower BMIs and younger ages than in European populations, or if you have a family history. Run it alongside fasting glucose and read it next to your ferritin, not on its own. Running it every quarter because a package deal includes it produces noise.
Thyroid panels: when they're worth running
A thyroid panel earns its place when there are symptoms a clinician wants to investigate, and a low T3 in a hard-training athlete is more often a fuelling problem than thyroid disease. Low energy availability suppresses T3 as an adaptive response, and the IOC consensus on Relative Energy Deficiency in Sport lists reduced T3 among the endocrine markers of the condition (Mountjoy et al., BJSM, 2023).
The distinction matters because the treatments are opposite. Thyroid disease gets treated by an endocrinologist. Energy deficiency gets treated by eating more, consistently, for months. Thyroid hormone prescribed to an under-fuelled athlete addresses the reading rather than the cause, and it's also a banned substance route in tested sport.
Get a panel if you have symptoms, a family history, or a clinician's reason. Don't add TSH to a routine annual out of curiosity, because a mildly odd result in an athlete training hard on too little food sends you down the wrong road entirely.
CRP and inflammation markers
CRP is worth measuring when you have a reason to suspect something is wrong, and worth ignoring the rest of the time. Hard training raises it transiently, so a single elevated reading in an athlete who trained the day before means very little.
Where it earns its place is alongside ferritin. Ferritin is an acute-phase reactant, meaning inflammation pushes it up, so a ferritin of 45 ng/mL with a raised CRP may be masking genuinely low stores. Read together, the pair tells you something neither tells you alone.
What athletes shouldn't be testing
Food-sensitivity IgG and IgG4 panels have no evidence base and should not be part of an athlete's testing. The joint AAAAI–EAACI PRACTALL update lists allergen-specific IgG and IgG4 testing among the modalities lacking sufficient evidence to recommend (Sampson et al., Pediatr Allergy Immunol, 2024), which restates a position the two academies have held since the original task force report (Stapel et al., Allergy, 2008) and which Canadian guidance reached independently, noting that positive results are expected in healthy people (Carr et al., Allergy Asthma Clin Immunol, 2012). What these panels reliably produce is an unnecessary elimination diet, which for an athlete usually means eating less than they need.
Routine testosterone panels in healthy young male athletes without clinical indication belong in the same category. So does any panel sold as a package by a supplement company where the recommended next step is a product from the same company.
Fueletics Perspective
The order matters more than the panel. Test because a symptom or a risk factor gave you a reason, read the result against athlete-relevant ranges rather than the lab's general-population column, fix what's low with food where food can do it, and supplement only where a deficiency has been confirmed. That last step is where most athletes start, and it's why so many end up with a shelf of products and the same heavy legs. The supplements post covers the rest of that argument.
The pattern I see most often isn't a deficiency at all. It's an athlete eating too little for their training load, feeling flat, and going looking for a marker to explain it. Chronic under-fuelling doesn't announce itself as one out-of-range number. It shows up as a scatter of soft abnormalities across iron, thyroid and hormones, which is exactly why an energy availability problem gets mistaken for four separate ones. No supplement resolves it. Only eating more does.
Your training load, bodyweight, sport and dietary pattern set the intake you actually need, and no blood test calculates that for you. If you want those numbers worked out properly before you go looking for something to fix, start with a Fueletics assessment.
FAQ
How often should athletes get blood tests?
Once a year is enough for most amateur athletes with no symptoms and no risk factors. Twice a year is reasonable for female endurance athletes and vegetarian or vegan athletes, because iron and B12 are the two things most likely to drift. Beyond that, test when something changes, not on a calendar.
What blood tests should a runner get?
Ferritin, full blood count, vitamin D, and B12 if vegetarian, with RBC magnesium if cramping or fatigue is part of the picture. That covers most of what changes a runner's plan. Adding markers beyond this rarely produces an action.
What is a good ferritin level for athletes?
Most sports medicine guidance puts the floor for endurance athletes at 30–35 ng/mL, with 40–50 ng/mL preferred, well above the roughly 15 ng/mL that labs use to flag general-population deficiency (Solberg & Reikvam, Life, 2023). A "normal" lab result can still be low for someone training hard.
Why is my HbA1c high if I exercise a lot?
Iron deficiency is one of the more common reasons. It extends red blood cell survival, which gives glucose longer to bind and raises HbA1c without any change in blood sugar (Radin, J Gen Intern Med, 2014). Check ferritin before accepting a borderline HbA1c at face value.
Should I test serum magnesium or RBC magnesium?
RBC magnesium. Serum holds around 1% of body magnesium and stays in range while tissue stores fall, so it can read normal during genuine depletion (Costello & Nielsen, Curr Opin Clin Nutr Metab Care, 2017). Ask the lab for the red blood cell test specifically, since it isn't standard on most panels.
Research References
Solberg A, Reikvam H. Iron Status and Physical Performance in Athletes. Life. 2023;13(10):2007. doi:10.3390/life13102007
Farrokhyar F, Tabasinejad R, Dao D, et al. Prevalence of Vitamin D Inadequacy in Athletes: A Systematic Review and Meta-Analysis. Sports Med. 2015;45(3):365–378. doi:10.1007/s40279-014-0267-6
Naik S, Mahalle N, Bhide V. Identification of vitamin B12 deficiency in vegetarian Indians. Br J Nutr. 2018;119(6):629–635. doi:10.1017/S0007114518000090
Costello RB, Nielsen F. Interpreting magnesium status to enhance clinical care: key indicators. Curr Opin Clin Nutr Metab Care. 2017;20(6):504–511. doi:10.1097/MCO.0000000000000410
Radin MS. Pitfalls in Hemoglobin A1c Measurement: When Results May Be Misleading. J Gen Intern Med. 2014;29(2):388–394. doi:10.1007/s11606-013-2595-x
Mountjoy M, Ackerman KE, Bailey DM, et al. 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). Br J Sports Med. 2023;57(17):1073–1097. doi:10.1136/bjsports-2023-106994
Sampson HA, Arasi S, Bahnson HT, et al. AAAAI–EAACI PRACTALL: Standardizing oral food challenges—2024 Update. Pediatr Allergy Immunol. 2024;35(11):e14276. doi:10.1111/pai.14276
Stapel SO, Asero R, Ballmer-Weber BK, et al. Testing for IgG4 against foods is not recommended as a diagnostic tool: EAACI Task Force Report. Allergy. 2008;63(7):793–796. doi:10.1111/j.1398-9995.2008.01705.x
Carr S, Chan E, Lavine E, Moote W. CSACI Position statement on the testing of food-specific IgG. Allergy Asthma Clin Immunol. 2012;8(1):12. doi:10.1186/1710-1492-8-12
