Platelets: what the test shows and why it matters for athletes

The platelet count is one of the lines in a complete blood count that athletes pay attention to less often than hemoglobin or hematocrit. Yet it is precisely these cells that are responsible for stopping bleeding, take part in healing microtraumas and respond to intense loads. The editorial team explains what the test measures, how to prepare for it correctly and why the indicator matters for people who train regularly.

What platelets are and where they come from

Platelets, or blood plates, are the smallest formed elements of the blood. Unlike red and white blood cells, they are not full-fledged cells: they are fragments of the cytoplasm of large bone marrow cells — megakaryocytes. A single megakaryocyte is able to form thousands of plates, which enter the bloodstream and circulate there for about a week to a week and a half.

The main regulator of platelet formation is the hormone thrombopoietin, which is synthesized primarily by the liver. When there are fewer plates in the blood, the amount of free thrombopoietin increases, and the bone marrow steps up their production. This feedback mechanism keeps the platelet count within relatively stable limits in a healthy person.

At any given moment about a third of all the body's platelets are not in circulation but in the spleen. This reserve has practical significance for athletes: during intense physical exertion, under the action of adrenaline, the spleen contracts and releases some of the plates into the blood, which temporarily raises their concentration in the test.

The main function of platelets is primary hemostasis. At the site of a vessel injury they adhere to the exposed collagen, become activated, change shape and clump together, forming a «plug». In parallel they release substances that trigger the coagulation cascade and attract cells that take part in tissue repair, in particular growth factors.

That is why the number and function of platelets matter not only to surgeons and hematologists. For a person who gets bruises, abrasions and microtears of muscles and tendons, normal platelet function is part of the ordinary recovery process after training and competition.

Which platelet indicators are in the test

In a complete blood count, modern hematology analyzers usually report not one but several parameters related to platelets. The most important of them is the absolute number of plates (PLT); the rest help the doctor understand exactly how the bone marrow produces them and how uniform they are in size.

Reference ranges should always be checked against the form of the specific laboratory: they depend on the measurement method, the analyzer model and the population on which they were calculated. Results obtained in different laboratories should be compared with caution, and for dynamic monitoring it is better to give blood in one place.

Mean platelet volume (MPV) indirectly reflects their «age»: young plates, just released by the bone marrow, are generally larger. Therefore a combination of a low count with a high MPV may indicate increased destruction of platelets in the periphery, while a low count with a low MPV points rather to insufficient production.

At the same time, MPV, PDW and plateletcrit taken separately have limited diagnostic value and easily «drift» because of how long the tube is stored before analysis. The editorial team advises not drawing conclusions from one of these auxiliary indicators if the main one — the platelet count — is normal.

How training affects platelets

Physical exertion is one of the best-known factors that temporarily change blood indicators. Immediately after intense work the platelet count rises: some of the plates leave the spleen and other reserves, and because of fluid loss from the plasma the blood becomes more concentrated. This effect is described, in particular, in observations of marathon participants (Kratz et al., 2002).

Usually this «athletic» rise disappears within a few hours or a day of rest, and on a morning test after a day without training the indicator returns to the person's usual level. This is a normal physiological reaction, not a sign of illness, but it is precisely what most often distorts the result if blood is taken immediately after training.

Intense exertion also temporarily enhances platelet activation — their ability to clump in response to a stimulus. A review by Heber and Volf (2015) showed that acute exhausting exertion in untrained people can increase the reactivity of the plates, whereas regular moderate training, on the contrary, is associated with a milder platelet response at rest.

A separate factor is dehydration. During prolonged work in the heat, «cutting weight» before competition or inadequate drinking, blood plasma decreases, and all cellular indicators, including platelets, formally rise. A doctor assessing an athlete's test always takes into account the hydration state in which the blood was drawn.

Why the test matters specifically for athletes

The first reason is safety in contact sports. A significant drop in platelets increases the risk of bleeding: bruises appear from minimal blows, gums bleed, and blood from cuts takes longer to stop. For a boxer, wrestler, footballer or hockey player, pronounced thrombocytopenia is a risk of serious internal hemorrhage, and the decision on clearance is made by a doctor.

The second reason is platelets as an «indicator» of other processes. An increase in their number may result from chronic inflammation, hidden blood loss or iron deficiency — a condition very common among endurance runners, female athletes and vegetarians. That is why the indicator is assessed together with hemoglobin, ferritin and the white blood cell differential.

The third reason is monitoring against a background of medications. A number of drugs that athletes may take as prescribed by a doctor or on their own affect the plates: non-steroidal anti-inflammatory drugs suppress their function (the count does not change in this case), while some antibiotics and other drugs can reduce the count. A regular test helps to notice such changes in time.

• Pre-season screening:a baseline complete blood count gives a «reference point» for later comparisons.
• Frequent bruises, nosebleeds:grounds for an unscheduled check of platelets and a coagulation panel.
• A past viral infection:some viruses temporarily reduce the number of plates.
• Taking drugs that affect the blood:monitoring on a doctor's recommendation.

It is worth remembering that the test assesses the quantity, not the quality, of the plates. A person with a normal PLT may have impaired platelet function — because of medications or hereditary conditions. If there is a tendency to bleeding while the count is normal, a doctor may order additional studies of platelet function and coagulation.

How to take the test correctly

So that the result reflects the real state rather than the consequence of yesterday's training, the editorial team advises following a few simple rules. They coincide with the general recommendations for the pre-analytical stage described, in particular, by Sanchis-Gomar and Lippi (2014), who emphasize that physical activity is one of the most important variables before blood is drawn.

1. Do not perform intense training for at least 24 hours, and after very prolonged loads — longer.
2. Give blood in the morning, after a night's sleep, in a calm state; sit for 10–15 minutes before the procedure.
3. Drink water as usual, avoiding both dehydration and excessive drinking immediately before the test.
4. Tell the doctor about all medications and supplements you take, especially anti-inflammatory drugs.
5. For comparison over time, use a single laboratory.

Sometimes the analyzer shows a falsely low platelet count because of a laboratory artifact — so-called pseudothrombocytopenia. In some people the anticoagulant EDTA, which standard tubes contain, causes the plates to clump in the tube, and the device counts the clumps as one platelet or does not count them at all. This condition is not a disease and does not increase the risk of bleeding.

If a low result turned out to be unexpected while there are no symptoms, the doctor usually asks for the test to be repeated, sometimes in a tube with a different anticoagulant (for example, citrate), and to review the blood smear under a microscope. Such a simple step avoids needless worry and unnecessary examinations.

Editorial conclusions

Platelets are small but vital elements of the blood, responsible for stopping bleeding and taking part in healing injuries. A complete blood count shows their number and several auxiliary parameters, of which the main one for assessment is PLT.

In athletes the indicator is sensitive to the last training session and to hydration: after exertion the count temporarily rises. Therefore blood for assessment should be given in a rested state, in the morning, with a break in training, and results should be compared over time in a single laboratory.

Persistent deviations, especially together with bruises, bleeding, fatigue or other changes in the test, are grounds for seeing a doctor rather than looking for an explanation on one's own. For contact sports this is also a matter of safety.

The editorial team also recommends reading our materials on why platelets can be higher or lower than normal, on white blood cells in an athlete's test, and on ferritin and iron stores.

References

1. Kratz A, Lewandrowski KB, Siegel AJ, et al. Effect of marathon running on hematologic and biochemical laboratory parameters, including cardiac markers. Am J Clin Pathol. 2002;118(6):856–863.
2. Heber S, Volf I. Effects of physical (in)activity on platelet function. Biomed Res Int. 2015;2015:165078.
3. Sanchis-Gomar F, Lippi G. Physical activity – an important preanalytical variable. Biochem Med (Zagreb). 2014;24(1):68–79.
4. Schafer AI. Thrombocytosis. N Engl J Med. 2004;350(12):1211–1219.
5. Neunert C, Terrell DR, Arnold DM, et al. American Society of Hematology 2019 guidelines for immune thrombocytopenia. Blood Adv. 2019;3(23):3829–3866.
6. Kaushansky K, Lichtman MA, Prchal JT, et al (eds). Williams Hematology. McGraw-Hill Education.