Blood tests for runners: what to check before the season and after a marathon

Regular running changes your body’s biochemistry more than most amateur runners realise. Increasing mileage means a greater need for iron, faster use of magnesium and potassium, strain on muscles and kidneys, and fluctuations in stress hormones. Most of these changes are physiological and reversible. Problems begin when they accumulate over weeks and nobody measures them. Below is a list of blood tests worth having before the season, midway through your training cycle and after a marathon, with a brief explanation of what each one is for.

Why runners should have blood tests more often than people who do not train

Endurance training affects several body systems at once. Foot strikes against the ground destroy some red blood cells (known as foot-strike haemolysis), sweating depletes electrolytes, and long runs raise markers of muscle breakdown. Added to this are a reduced appetite after hard sessions and, for many people, calorie restriction to reach their “race weight”. The result: runners are more likely than the general population to have low ferritin, vitamin D and magnesium deficiencies and, with overtraining, hormonal disturbances too.

Symptoms develop slowly: poorer recovery, a higher heart rate at the same pace, irritability, infections and a lack of progress despite a well-structured plan. A blood test helps distinguish ordinary fatigue from a deficiency that can be corrected within a few weeks.

Full blood count with differential: a foundation for every athlete

A full blood count shows the number and characteristics of red blood cells (oxygen transport), white blood cells (immunity) and platelets (clotting). For runners, the key parameters are haemoglobin (Hb), haematocrit (Hct), MCV, MCH and RDW. Low haemoglobin with a normal erythrocyte count and low MCV usually indicates iron deficiency. Raised RDW is an early sign that the bone marrow is producing blood cells of uneven size, which precedes overt anaemia.

Be aware of so-called sports pseudoanaemia: in people who train intensively, plasma volume increases, so haemoglobin may be slightly “diluted”. This does not require treatment, but it needs to be distinguished from a genuine deficiency. That is why it is always worth assessing a full blood count alongside ferritin.

Ferritin and iron: runners’ most common hidden problem

Ferritin reflects the body’s iron stores. In runners, especially women and people on plant-based diets, it is the first to fall, long before changes appear in the full blood count. Many sports medicine specialists consider that, in endurance athletes, values below 30 ng/ml (and, according to some centres, below 50 ng/ml) already affect performance, even though the laboratory reference range may be wider. Reference ranges vary between laboratories, so a result is always interpreted against the stated range and your symptoms. The result report is issued in Polish.

Ferritin is also an acute-phase protein: it rises during inflammation and after hard training. It is best tested after 24–48 hours of rest and alongside CRP, to rule out a misleadingly normal result.

Vitamin D (25-OH-D3)

In Poland, vitamin D production in the skin virtually stops from October to April. Vitamin D deficiency is associated with poorer muscle strength, slower recovery and greater susceptibility to upper respiratory tract infections, which are already more common in runners after long runs. One measurement in autumn and another in spring allow you to choose a supplement dose rather than guess.

Electrolytes: sodium, potassium, magnesium, calcium

Muscle cramps after training, palpitations and a feeling of “heavy legs” are common reasons for questions about electrolytes. Potassium and magnesium are lost through sweat and urine, and with high training volumes, replacing them through diet alone can be difficult. Serum sodium is important for people taking part in ultramarathons, where drinking too much plain water can lead to hyponatraemia. Calcium, in turn, helps complete the picture of bone health, which is important for female runners with menstrual disturbances.

Creatine kinase (CK) and liver function tests

CK is an enzyme released from damaged muscle fibres. After a marathon, it can rise more than tenfold, and this is normal. Persistently high values despite several days of rest are concerning, as they indicate incomplete recovery or overtraining. ALT and AST (liver function tests) also rise after exercise because AST is also found in muscles. A doctor who does not know that a patient runs may wrongly suspect liver disease. It is therefore best to have the tests after 2–3 days without intensive training and mention your training when giving your medical history.

Creatinine, urea, uric acid: kidneys under strain

Creatinine depends on muscle mass and hydration, so it can be slightly higher in athletes. Markedly raised creatinine and urea after a long run in hot weather signal dehydration and excessive strain on the kidneys. Uric acid rises with high-protein diets and intensive cell breakdown; monitoring it is important for people with joint pain or a family history of gout.

Glucose, lipid profile, CRP

Fasting glucose and a lipid profile (total cholesterol, LDL, HDL, triglycerides) are preventive tests that runners have for the same reasons as any adult. Regular training usually improves HDL and lowers triglycerides, so the result also provides feedback on the effects of physical activity. The high-sensitivity version of C-reactive protein (CRP) testing shows chronic, low-grade inflammation; a raised level in the absence of infection suggests that the body is struggling to keep up with recovery.

Hormones: testosterone, cortisol, TSH

With high training volumes and an energy deficit, the body conserves resources: testosterone falls (in both sexes), cortisol rises, and women stop having periods. This is relative energy deficiency in sport (RED-S), which leads to reduced bone density and stress fractures. Measuring testosterone and cortisol in the morning on an empty stomach, plus thyroid-stimulating hormone (TSH) to rule out an underactive thyroid (which also causes fatigue and weight gain), helps identify the problem before it results in injury.

When to test and how to prepare

• Before the season (the base training phase): a full set of tests, so you can start with any deficiencies corrected.
• After 8–12 weeks of increased training volume: full blood count, ferritin, CK and magnesium; a check that your plan is not exceeding your capacity to recover.
• After a marathon or ultramarathon: wait 5–7 days, otherwise CK, ALT, creatinine and ferritin will be distorted by exercise.
• Preparation: have blood collected in the morning, on an empty stomach (12 hours without food, water is allowed), after a good night’s sleep, with no intensive training for 48 hours and no alcohol for 2–3 days; discuss your regular medicines with your doctor.

How to read your results without panicking

A single deviation of a few per cent from the reference range rarely means disease. The trend matters: ferritin falling from 60 to 25 ng/ml over a season tells you more than a one-off result. It is worth keeping your results in one place (most laboratories make them available online) and comparing them year on year. If your results are clearly outside the reference range, especially haemoglobin, potassium, creatinine or TSH, see a doctor; this article does not replace a medical consultation.

Frequently asked questions

Do runners need a referral for blood tests?

No. All the tests listed can be done privately without a referral: buy them online and attend your chosen collection point with an identity document.

How much does a set of tests for runners cost?

The current price of the basic set (full blood count, ESR, glucose, urinalysis) is shown on BadamySie.pl, as is the price of a full blood count with differential alone in the ALAB, Synevo and ETER-MED networks. Current prices for comprehensive packages including hormones and electrolytes are also shown on BadamySie.pl.

How often should ferritin be retested?

Once a year if the result is normal; if you take iron supplements, retest after 8–12 weeks to assess effectiveness and avoid taking too much.

Sources

• Sim M. et al., Eur J Appl Physiol, Iron considerations for the athlete: a narrative review, 2019. link.springer.com
• Kaleta-Duss A. et al., Int J Environ Res Public Health, Myocardial Injury and Overload among Amateur Marathoners as Indicated by Changes in Concentrations of Cardiovascular Biomarkers, 2020. pmc.ncbi.nlm.nih.gov
• Clénin G. et al., Swiss Medical Weekly, Iron deficiency in sports: definition, influence on performance and therapy, 2015. doi.org
• Maughan RJ et al., Br J Sports Med, IOC Consensus Statement: Dietary Supplements and the High-Performance Athlete, 2018. bjsm.bmj.com