Which Blood Tests Do Athletes Actually Need? A Doctor's Evidence-Based Guide

Introduction

It’s completely understandable that athletes want to maximize their performance. That’s only natural. They already invest significant time and effort into their training, recovery, and nutrition, so it makes sense to also want to minimize any deficiencies or other factors that could negatively impact performance.

The problem is that the internet is full of misinformation, social media influencers, bro science, and oversimplified advice. As a result, many athletes come to believe that staying on top of their health requires repeated, expensive laboratory panels.

In reality, this is usually not necessary.

The uncomfortable truth is that more testing does not equal better information. A systematic review published in BMJ Open evaluated 484 direct-to-consumer tests and found that only 10.7% had genuine clinical utility — 41.9% were non-evidence-based commercial health checks, 30.6% had limited utility, and 16.7% used methods or tested for conditions not recognised by the general medical community [1].

When enough markers are tested, one or more will often fall outside the reference range purely by chance. Under a simple independence assumption, a 20-marker panel would produce at least one “abnormal” result about 64% of the time, even in a perfectly healthy individual. Each out-of-range value then creates an invitation for anxiety, follow-up testing, and sometimes unnecessary interventions [1][2].

This article is not a list of tests to order. Instead, it provides a clinical framework for understanding which blood tests are genuinely useful for athletes, which are only useful in specific contexts, and which produce numbers that appear meaningful but offer little actionable value.

In Finnish healthcare, there is a strong emphasis on avoiding unnecessary testing. This is partly driven by the structure of our system, where a strong public sector encourages cost-conscious decision-making. But it also reflects something more practical.

Many athletes operate on limited budgets. They are often students, reliant on sponsorships, grants, or unstable income streams. Every euro matters.

That is exactly why I wrote this article — for those who want to take care of their health and performance, but do not want to waste money on tests that offer little real value.

The Foundational Question: What Makes a Blood Test Useful?

Before discussing individual markers, it is worth establishing what actually makes a blood test worth ordering. Clinically, a useful test must satisfy three conditions: it measures something with a reliable method, the result changes what you do, and doing that thing produces a better outcome. A test that meets the first condition but not the second or third generates information without value — and information without value in medicine is not neutral, because it creates the opportunity for misinterpretation, anxiety, and unnecessary intervention [1][2].

In an athletic context, there is a fourth condition that most commercial panels ignore entirely: the result must be interpreted against an athlete-appropriate reference range. Population-based laboratory reference ranges are built from general, largely sedentary populations and consistently misclassify normal athletic physiology as pathological [3][4]. This is not a minor caveat — it is the reason that a large proportion of flagged values on athlete blood panels represent adaptation, not disease.

At the same time, healthcare providers are increasingly marketing broad laboratory panels directly to consumers, sometimes approaching an ethical grey area.

In clinical practice, ordering tests should go hand in hand with interpreting them. Ideally, the same clinician who orders the tests is also responsible for reviewing and acting on the results. This creates accountability and helps reduce unnecessary testing.

When the person ordering the tests is also responsible for the consequences, the threshold for ordering low-value investigations is naturally higher.

Which Blood Tests Do Athletes Actually Need: Tests With Genuine Clinical Utility

These are the markers where the evidence is strong, the methodology is reliable, the result changes clinical decisions, and the change in management produces better outcomes. These are also the same tests I order regularly in my own clinical practice. In fact, they are largely the same core investigations I use for my patients regardless of whether they are athletes or not.

Complete Blood Count (CBC)

The CBC is the most clinically justified routine test in athletes. It screens for anaemia, monitors the haematological adaptations of endurance training, and provides early signals of iron deficiency, infection, and overreaching through white blood cell changes. In athletes, these values must be interpreted with training context in mind: endurance training expands plasma volume and can lower relative haemoglobin even as absolute oxygen-carrying capacity improves — this is adaptation, not pathology (see haemoglobin levels in runners and RBC count in endurance athletes). White blood cell count elevates transiently post-exercise and may mislead clinicians who test too soon after training (see high white blood cell count after exercise).

The CBC earns its place because the conditions it identifies — iron deficiency anaemia, infection, significant haematological abnormality — are common, clinically significant, and respond to treatment. It is inexpensive, widely available, and provides multiple data points from a single draw.

In my own clinical practice, I order a CBC alongside most other laboratory tests. It is one of the most frequently requested investigations in my workflow. I only rarely omit it, usually for cost-related reasons — although in reality, it is not a particularly expensive test.

Iron Panel: Ferritin, Serum Iron, TIBC, Transferrin Saturation

Iron status is the most clinically consequential nutritional biomarker in endurance athletes. Iron deficiency — even without overt anaemia — impairs mitochondrial function, oxygen transport, and exercise economy in ways that are directly and measurably reversible with treatment [6]. Standard laboratory ferritin thresholds (commonly 12–15 µg/L) were derived from non-athletic populations. In sports medicine practice, ferritin below 30–40 µg/L is commonly used as a clinically meaningful threshold for athletes, reflecting the higher iron demands of training — though the exact cutoff is not universally settled in the literature and should be interpreted alongside the full iron panel and clinical picture [6].

Ferritin should never be interpreted in isolation. It is a positive acute-phase reactant — it rises with inflammation, potentially masking true iron depletion and giving false reassurance. A concurrent CRP is essential for accurate interpretation (see ferritin levels for athletes and TIBC in athletes). For athletes with repeatedly ambiguous panels, soluble transferrin receptor (sTfR) adds genuinely discriminating information because it is unaffected by inflammation (see sTfR as the advanced iron marker).

Ferritin is already familiar to many athletes. In my clinical work, I often find that athletic patients specifically ask for it — and for good reason. As discussed earlier, ferritin levels can influence performance, particularly in the context of iron deficiency.

In recent years, ferritin has also become something of a “trend” laboratory marker. Many patients presenting with fatigue suspect that low iron levels may be the underlying cause. While this is sometimes correct, it is important to approach the diagnosis with care. Fatigue is a highly multifactorial symptom, and iron deficiency is only one of many possible contributors.

In my practice, I frequently include ferritin as part of the initial evaluation of fatigue. However, the result should always be interpreted alongside the full clinical picture and other laboratory findings. Only after considering all relevant factors can fatigue be reasonably attributed to iron deficiency.

Vitamin D (25-OH Vitamin D)

Vitamin D insufficiency is highly prevalent in athletic populations worldwide — a 2022 systematic review and meta-analysis found it common among elite athletes [8]. The clinical consequences are meaningful: deficiency is associated with impaired muscle function, elevated stress fracture risk, and compromised immune defence. A 2024 systematic review of randomised controlled trials in elite athletes found that vitamin D supplementation may improve aerobic endurance, anaerobic power, and strength — though the authors note the evidence is not yet definitive and more research is needed, particularly on bone health and injury risk [7]. What is well established is that deficiency is prevalent, measurable, and correctable (see vitamin D for athletes).

This test earns its place because the prevalence is high, the consequences are significant, and the intervention — supplementation — is safe, inexpensive, and supported by a growing evidence base.

Vitamin D deficiency is highly prevalent, particularly in northern countries such as my home country, Finland. Due to the high latitude, sunlight exposure is limited for a significant part of the year, and during the winter months, very little vitamin D is produced in the skin.

For this reason, I often recommend vitamin D supplementation to my patients, especially during the darker months.

CRP (C-Reactive Protein)

CRP on its own is not a performance optimization tool. Its value in the athletic context is almost entirely as an interpretive key for other markers. Because ferritin, white blood cell count, and several other markers are acutely elevated by exercise-induced inflammation, a CRP drawn at the same time provides essential context for deciding whether an abnormal result reflects training load, active infection, or genuine pathology. CRP rises approximately 24 hours after strenuous endurance exercise and returns to baseline within 48 hours in healthy athletes — which is why timing of the blood draw relative to the last hard session matters considerably (see inflammation markers in athletes).

CRP is a commonly used laboratory marker in my clinical practice. In many cases, however, its role is primarily to help rule out underlying inflammation rather than to confirm a specific diagnosis.

Symptoms such as fatigue, reduced performance, and general malaise can sometimes be linked to inflammatory processes, which makes CRP particularly useful in this context. In healthy individuals, CRP levels are typically low or undetectable. High-sensitivity CRP (hs-CRP) can provide more precise measurements at lower ranges, but even then, its primary value in this setting remains in excluding clinically significant inflammation rather than diagnosing a specific cause.

Thyroid Function (TSH and T4-V)

Hypothyroidism can present with a clinical picture that closely resembles overtraining syndrome — including fatigue, reduced performance, mood changes, and cold intolerance. Because these conditions can be difficult to distinguish, and because hypothyroidism is both relatively common and treatable, assessing thyroid function is a reasonable part of the initial evaluation when an athlete presents with unexplained fatigue, particularly in female athletes who are at higher baseline risk (see female athlete bloodwork) [14].

More broadly, thyroid function tests are often included in the early assessment of symptoms related to metabolism, energy levels, fatigue, or performance. In my clinical practice, I order them regularly, regardless of whether the patient is an athlete or not. This reflects the fact that thyroid dysfunction can manifest through a wide range of non-specific symptoms, making it an important condition to consider early in the diagnostic process.

It is worth noting that this approach is not strongly dictated by sports medicine guidelines, but instead reflects my clinical judgement — the cost of missing hypothyroidism is high, while testing is inexpensive and low-risk.

Tests That Are Useful — But Only in the Right Context

These markers have genuine scientific rationale but are frequently ordered without the conditions that make them interpretable. Used correctly, they add value. Used as routine screening, they generate numbers that are almost impossible to act on appropriately.

Testosterone and Cortisol (T:C Ratio)

The testosterone-to-cortisol ratio has been proposed as a serial monitoring tool for detecting functional overreaching in competitive athletes, and it has a reasonable mechanistic basis — testosterone is broadly anabolic, cortisol broadly catabolic, and the balance between them reflects physiological stress [9]. A decline of ≥30% from an individual’s personal baseline has been suggested as a clinically relevant signal, though this threshold has been criticised in the literature and a decline of this magnitude does not reliably predict performance deterioration in all contexts [9]. The practical consensus is that serial monitoring of the T:C ratio has potential utility — but that utility is modest, debated, and entirely dependent on having established individual baselines under standardised conditions.

A single cortisol measurement in isolation is essentially uninterpretable for monitoring purposes in a healthy athlete. Cortisol follows a powerful diurnal rhythm, with levels surging approximately 50–60% in the first 30–40 minutes after waking and declining through the day. One study found that time of day accounted for approximately 72% of the variance in salivary cortisol levels — and while the precise figure varies across measurement methods and populations, the broader point holds: without standardised timing, a cortisol result primarily tells you what time the blood was drawn [11]. For serial monitoring to be meaningful, every draw must occur at the same time of day, in the same fasted state, under similar training conditions. This is rarely achieved in commercial testing contexts. See T:C ratio in overtraining and cortisol and overtraining for the full clinical framework.

The T:C ratio is not routinely available as a standard laboratory parameter. In practice, cortisol is rarely measured outside specific endocrinological indications — at least in Finland — whereas testosterone is tested more frequently, particularly in male patients.

In theory, because the T:C ratio is largely influenced by testosterone levels, measuring testosterone alone may capture part of the same signal. However, the same limitations apply. For any meaningful interpretation, individual baseline levels must be known, and measurements need to be taken repeatedly under standardized conditions — at the same time of day and in a comparable physiological state.

Importantly, testosterone levels on their own have limited diagnostic value in this context. Outside of clear and persistent testosterone deficiency suggestive of hypogonadism, isolated measurements rarely provide actionable information regarding performance, recovery, or training status.

Creatine Kinase (CK)

CK is a genuine marker of exercise-induced muscle damage and has real utility for monitoring training load in structured programmes [4][10]. Upper reference limits in trained athletes are roughly twice those of non-athletes [5], meaning a CK of 600 U/L in a male strength athlete during a heavy training block may represent entirely normal physiology. What matters is not any single value but the trend: a progressive rise in resting CK across consecutive weeks signals accumulating muscle damage that may warrant load reduction. A single draw, without context and without a personal baseline, tells you very little (see creatine kinase elevated in athletes).

In my clinical practice I rarely see a strong indication to measure CK outside specific scenarios. It can be useful when there is a suspicion of rhabdomyolysis, or in rare cases when evaluating certain neuromuscular conditions.

Beyond that, CK has limited value as a routine marker for performance or general health in most athletes. Levels can vary widely depending on recent training load, making single measurements difficult to interpret in a meaningful way. For this reason, I generally do not see a strong rationale for measuring CK in the absence of a clear clinical indication.

Complete Metabolic Panel, HbA1c, Lipids

These are appropriate for baseline health screening and cardiac risk assessment, particularly for masters athletes or anyone with relevant family history or symptoms. They are not performance optimisation tools for healthy athletes under 40 without risk factors. The value is in identifying clinically significant metabolic abnormalities that exist independent of training — not in explaining a slow race time.

These tests are, of course, part of a broader assessment of baseline health and risk factors. That said, athletes are generally at lower risk for metabolic diseases. However, this is not universally the case — some individuals may still have suboptimal dietary habits or other risk factors, regardless of their level of physical activity.

Tests With Limited Clinical Utility in Most Athletes

These markers are frequently included in commercial “athlete panels.” Their inclusion creates the impression of comprehensiveness while adding little actionable information for the majority of people who purchase them.

Serum Magnesium

Magnesium plays an important role in neuromuscular function and overall health, but its role in routine testing and supplementation is often misunderstood. One key issue is that serum magnesium is a poor marker of total body magnesium status. Approximately 99% of magnesium is stored intracellularly in bone and muscle, with less than 1% circulating in the blood [12]. Because the body tightly regulates serum magnesium through renal excretion and bone mobilisation, levels can remain normal even when intracellular stores are meaningfully depleted.

This means that a normal serum magnesium result does not reliably exclude deficiency, particularly in heavily training or symptomatic individuals. At the same time, low serum magnesium is relatively uncommon and typically reflects more significant deficiency states with identifiable causes.

Alongside this, a common myth — particularly in Finland and one I frequently encountered during my own athletic career — is that magnesium supplementation is necessary to prevent muscle cramps, or that increased muscle activity inherently increases magnesium requirements. In practice, this is rarely the case. Current evidence does not strongly support magnesium deficiency as a common cause of muscle cramps in otherwise healthy individuals, nor does it show that supplementation meaningfully reduces cramp frequency in most athletes.

Taken together, while magnesium is physiologically important, routine testing and supplementation without a clear clinical indication are unlikely to provide significant benefit.

Serum Zinc

The same logic applies to zinc. Serum zinc levels may not accurately reflect zinc deficiency or tissue status, and exercise itself acutely alters serum zinc concentrations, adding a further layer of confounding [13]. A serum zinc result in an otherwise healthy athlete is difficult to act on: it will not tell you whether supplementation is warranted, and supplementing without confirmed deficiency carries risk of interfering with copper absorption and immune function.

Zinc tends to come into focus periodically, particularly during cold and flu season. In my clinical practice, I often see patients start taking zinc supplements in an attempt to support their immune function and maintain performance.

In most cases, however, this is not necessary. Zinc deficiency is relatively uncommon in otherwise healthy individuals, and routine supplementation without a clear deficiency is unlikely to provide meaningful benefit.

Broad Hormone Panels for Recreational Athletes

Panels marketed as “full hormonal profiling” — typically including DHEA-S, SHBG, various oestrogen metabolites, prolactin, IGF-1, and cortisol alongside testosterone — are often aggressively targeted at recreational athletes. In the absence of specific symptoms, these results are difficult to interpret in a way that meaningfully changes management.

Clinical practice in sports endocrinology supports a more targeted approach, where hormonal testing is guided by symptoms and clinical suspicion rather than broad, untargeted panels. Markers such as SHBG, DHEA-S, and IGF-1 do have clear roles in specific scenarios — for example, suspected hypogonadism, evaluation of the female athlete triad, or unexplained fatigue with supporting clinical features. However, ordering them in asymptomatic individuals seeking “optimization” is not supported by clinical evidence.

A further issue is that without established personal baselines and appropriate clinical context, broad hormone panels often generate findings that are difficult to interpret. Statistically, when enough markers are measured, some values will fall outside reference ranges by chance alone — and the larger the panel, the more likely this becomes.

In my clinical practice, I frequently see male patients who have had their testosterone levels tested through commercial services. As a reactive hormone, testosterone can be temporarily reduced in the context of fatigue, stress, or insufficient recovery, making isolated measurements particularly unreliable.

In some cases, patients then pursue testosterone replacement therapy based on these findings, and in private settings, treatment may be initiated on relatively loose indications. However, low testosterone in this context does not necessarily indicate true androgen deficiency.

When treatment is initiated without clear diagnostic criteria, there is a real risk of iatrogenic harm. Exogenous testosterone can suppress endogenous production and impair fertility, meaning that patients may face unintended long-term consequences from treatment that was not clearly indicated.

Summary

In summary, most athletes do not need broad, repeated laboratory panels to support their health or performance. The most useful tests are typically the same core investigations used in everyday clinical practice: a complete blood count, iron studies, vitamin D, CRP, and thyroid function when clinically indicated. These tests are valuable because they help identify common, clinically meaningful, and treatable conditions that can genuinely affect health and performance.

By contrast, many commercially marketed panels include markers that are difficult to interpret, poorly suited to routine screening, or unlikely to change management in a meaningful way. Without clear clinical context, established baselines, and appropriate timing, these tests often produce results that create confusion rather than clarity — and may lead to unnecessary follow-up, treatment, or anxiety.

Ultimately, effective testing in athletes is not about measuring as much as possible, but about asking the right questions. Tests should be selected based on symptoms, clinical suspicion, and individual context, and interpreted with an understanding of normal physiological adaptations to training. In many cases, less testing — done thoughtfully — provides far more value than extensive panels driven by curiosity or marketing.

Which Blood Tests Do Athletes Actually Need? A Doctor’s Evidence-Based Guide

Table of Contents

Introduction

The Foundational Question: What Makes a Blood Test Useful?