Platelet Count for Runners: Understanding Blood Test Changes in Marathon Training
Table of Contents
Introduction
The interpretation of laboratory tests in athletes differs from interpretations in the general population. Often, abnormal laboratory findings in athletes are interpreted as pathological, even though they may be a completely physiological phenomenon. This phenomenon also applies to platelets, at least in the short term. Physical training causes changes in platelet count for runners, which may cause laboratory results to deviate from standard reference ranges without the presence of a real disease.
The standard clinical reference range defines normal platelet count as 150,000-450,000 platelets per microliter of blood[2]. Values below 150,000/µL constitute thrombocytopenia, while counts exceeding 450,000/µL indicate thrombocytosis. These values are defined purely statistically so that they fit within 95% of the general population. In other words, either below or above the reference values statistically fit within 2.5% of the normal population. However, these population-level reference values do not take into account the changes in platelets that occur in athletes.
Acute Changes in Platelet Count for Runners
Marathon runners’ platelet counts may vary greatly with exercise. Research demonstrates that platelet count runners show increases significantly immediately after marathon running[1]. This effect is usually acute, which is different from the long-term response.
A small study, conducted in my home country of Finland, examined the platelet counts of 10 Finnish marathon runners. The results showed that runners’ platelet count showed highly significant elevation post-race[1]. Because this acute response is different from the chronic adaptation, it often raises questions for athletes and even inexperienced physicians.
Long-term sports activities such as marathon running affect platelets through several mechanisms. In 32 healthy marathon participants, researchers demonstrated increases in hematocrit and platelet count. This acute increase in platelet number post exercise is primarily explained by dehydration. Dehydration causes a decrease in plasma volume, which in turn causes hemoconcentration, although total platelet mass remains unchanged[3].
However, alongside dehydration, there is also evidence of actual in vivo platelet activation[3]. This in vivo platelet activation is a physiological phenomenon; the body prepares itself for tissue damage. Partial activation of platelets increases the body’s readiness for potential trauma. Exercise transiently mobilizes platelet stores in the liver, lungs, and spleen[4] contributing to the acute thrombocytotic platelet response.
Hyperthermia increases this exercise induced thrombocytic effect, with hyperthermic conditions producing platelet counts of 274,000/µL compared to 246,000/µL in normothermic exercise and 196,000/µL in controls[5]. This study did not differentiate the mechanisms for the thrombocytic response.
In addition, one way of platelet activation is mechanical shear stress. Comparative analysis of 68 athletes across marathon, triathlon, and cycling revealed that elevated platelet aggregation occurred particularly during marathon and—to a lesser extent—triathlon, but not cycling. This finding suggests mechanical stress from prolonged running triggers platelet activation distinct from other endurance modalities[6].
These mechanisms, namely natural physiological platelet activation response, platelet mobilization from organs, dehydration, hyperthermia and shear stress explain why runners experience acute thrombocytic responses.
Chronic Changes in Platelet Count for Runners
Generally, in a chronic situation, studies show varying information about changes in platelet counts. Some studies show that after the acute phase, there is not much change from baseline, while others show slight decreases or increases.
However, in chronic training adaptation, platelet function changes clearly instead of the number. Eight weeks of exercise training in previously sedentary individuals reduced platelet sensitivity to ADP-induced aggregation and decreased agonist-induced platelet activation, demonstrating favorable functional adaptations[8]. So platelets are desensitized to avoid platelet hyperactivity related to physical exertion.
It is also known that stopping training reverses this effect. Cessation of regular exercise increases platelet aggregation and calcium elevation, confirming that adaptations require ongoing training stimulus[8].
The protective effects manifest through multiple pathways. Regular endurance exercise decreases resting and exercise-induced catecholamine levels, and trained athletes’ platelets demonstrate reduced sensitivity to adrenaline-induced aggregation compared to untrained individuals.
So acute and chronic thrombocyte effects are partly opposite: in an acute situation, platelets are more active and numerical, while chronic adaptations reduce their activity and do not necessarily affect their number. However, it is good to remember that the actual number of platelets does not seem to matter that much for a runner and that there is no evidence that the number of platelets affects an athlete’s performance[9].
Platelet Size Matters: MPV as Performance Predictor
Interestingly, mean platelet volume (MPV) seems to be an important parameter for runners, although as previously stated, the platelet count itself does not appear to be significant.
In a study of 43 amateur runners completing a half-marathon, pre-run MPV inversely correlated with running performance—lower MPV predicted faster finishing times[9]. This association remained significant after adjusting for age, sex, VO2max, training volume, and platelet count, confirming an independent relationship between platelet size and aerobic performance[9].
Interestingly, neither platelet count nor MPV at baseline correlated with VO2max, suggesting the platelet-performance relationship operates independently of cardiorespiratory fitness level[9]. The correlation between running time and pre-run MPV ranked second only to VO2max among all measured parameters[9].
However, it is likely that MPV is just a biomarker that correlates with performance as there is no direct evidence that MPV directly affects athletic performance. The exact mechanism is not known, but it is thought that a higher MPV could indicate platelet reactivity and thus reflect low-grade inflammation or poor vascular function and microcirculation.
What To Do With This Information?
It is important to recognise that long-term regular exercise does not usually cause permanent thrombocytosis or thrombocytopenia. Rather, acute exercise can cause transient thrombocytosis, but this should not be a permanent condition. Instead of quantity, exercise affects the activity level of platelets; in the short term, activity increases, while in the long term, it decreases their activity.
Persistent abnormalities in platelet levels in athletes usually indicate something other than the training itself. Therefore, if you as an athlete have thrombocytopenia or thrombocytosis, it is important to discuss the matter with your doctor, as it may well be that it is not caused by sports but may be related to other physiological processes or a disease.
References
1 https://pubmed.ncbi.nlm.nih.gov/578025/
2 https://www.ncbi.nlm.nih.gov/books/NBK542208/
3 https://pubmed.ncbi.nlm.nih.gov/16393676/
4 https://www.ncbi.nlm.nih.gov/books/NBK560810/
5 https://pubmed.ncbi.nlm.nih.gov/30231210/
6 https://pmc.ncbi.nlm.nih.gov/articles/PMC3352046/
