Bloodwork Basics

Hematocrit and Hemoglobin in Athletes: How to Read a CBC

Hydration, altitude, endurance training, and testosterone can all move hematocrit and hemoglobin. Learn what a CBC measures and why one number rarely tells the whole story.

By GetBloodWorx TeamPublished

The complete blood count (CBC) is one of the most commonly ordered blood tests, and for athletes it is often the one that raises the most questions. A hematocrit flagged high, or a hemoglobin sitting near the bottom of the range, can look alarming on a report. Both are concentrations, though, and concentrations depend on more than the number of red cells in the body.

This guide explains what the CBC measures, what tends to move the red-cell numbers in people who train, and where a result stops being a training artifact and starts being a question for a clinician.

What a CBC measures

A CBC counts and describes the cells in a blood sample. According to MedlinePlus, it reports:

  • Red blood cells, which carry oxygen from the lungs to the rest of the body.
  • Hemoglobin, the oxygen-carrying protein inside those cells.
  • Hematocrit, the percentage of blood volume made up of red blood cells.
  • Mean corpuscular volume (MCV), the average size of the red cells.
  • White blood cells, with a differential that breaks the total into each cell type.
  • Platelets, the cell fragments involved in clotting.

Hematocrit and hemoglobin usually move together, because they describe the same thing from two angles: how much of the sample is red cells, and how much oxygen-carrying protein those cells hold.

Why hematocrit is a ratio, not a count

Blood is cells suspended in plasma, the fluid portion. Hematocrit is the share taken up by red cells, so it changes when either side of that ratio changes.

That matters in practice. MedlinePlus lists dehydration as the most common cause of a high hematocrit: with less plasma, the same number of red cells takes up a larger fraction of the sample. The reverse also happens. Extra fluid lowers the percentage without any loss of red cells, which is why hematocrit is lower in pregnancy.

A single hematocrit therefore says something about red cells and something about fluid status on the morning of the draw, and it cannot separate the two by itself.

What can raise hematocrit and hemoglobin

Beyond dehydration, MedlinePlus describes several reasons the red-cell numbers can run high:

  • High altitude. Lower oxygen levels prompt the body to make more red blood cells.
  • Lung or heart disease, where the body compensates for poorer oxygen delivery.
  • Sleep apnea, listed among the possible causes of high hemoglobin.
  • Polycythemia vera, a condition in which the bone marrow makes too many red blood cells.
  • Smoking, which shifts the expected range.

Testosterone belongs on this list as well. The Endocrine Society’s clinical practice guideline on testosterone therapy names an elevated hematocrit among the conditions to evaluate before treatment and recommends that clinicians measure hematocrit as part of monitoring men who receive it. That is why a CBC so often travels with hormone testing; our guide to total and free testosterone covers the hormone side of that pairing.

Why endurance athletes can run low

Regular endurance training expands plasma volume. A review in Frontiers in Physiology describes how this accounts for the slightly lower hematocrit often seen in trained athletes, a pattern sometimes called “sports anemia.” The same review stresses that this is not clinical anemia: trained individuals tend to carry a larger total red-cell and hemoglobin mass, diluted in more plasma. Our guide to bloodwork for endurance athletes covers the other markers that shift with heavy training.

The picture is not uniform. The review notes that hematocrit also rises during exercise when fluid is not replaced, and that a share of athletes show high rather than low values. A low reading in someone who trains hard may be dilution, but it is not safe to assume so.

When low values point to something else

MedlinePlus lists the more familiar causes of low hemoglobin and hematocrit: anemia, including iron deficiency; blood loss, including heavy menstrual periods; and shortages of nutrients such as iron and vitamin B12.

Iron is the one athletes meet most often, and a CBC is a late signal for it. Iron stores can be depleted while hemoglobin still looks normal, which is the reason ferritin is usually read alongside the blood count. Ferritin and Iron for Athletes explains that relationship.

White cells and platelets

The rest of the CBC deserves a look too. MedlinePlus notes that a high white cell count may relate to infection or a reaction to a medicine, and that low counts can have several causes that need medical evaluation. Results outside the range do not always mean a condition that needs treatment: diet, activity level, medicines, menstrual periods, and not drinking enough water can all affect a CBC.

Getting a result you can compare

Because hydration moves the red-cell numbers, the conditions of the draw matter more for a CBC than its lack of a fasting requirement suggests:

  • Arrive normally hydrated, rather than straight from a hard or sweaty session.
  • Note recent altitude exposure, illness, blood donation, or heavy menstrual bleeding.
  • Keep the conditions similar from one test to the next, so a change reflects you and not the morning.

Our guide on how to prepare for a blood test covers the same idea across a full panel.

When to seek review

A high or low value that persists on a repeat test, changes substantially from your own baseline, or arrives with symptoms deserves a conversation with a qualified healthcare professional. Symptoms such as unusual fatigue, shortness of breath, dizziness, headaches, chest pain, or unexplained bruising or bleeding should be assessed promptly rather than explained away by training.

How an elevated hematocrit is managed, including whether any treatment that may be contributing should change, is a clinical decision. It is not something to adjust on the strength of an article or a single number.

Where the CBC fits

A CBC with differential is part of each of our panels, including Men’s Ultimate and Women’s Ultimate, and can be ordered individually on the build-your-own panel page.

Sources & further reading

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