A complete blood count measures three families of cells and reports a set of derived values for each. It is the most commonly ordered blood test, and most of what it reports is context rather than diagnosis.
⚠️ Reference ranges differ between laboratories because they depend on the analyser and on the local population. The range printed beside your own result is the one that applies. Figures below are typical adult values for orientation only.
White cells — infection and inflammation
| Value | Typical adult range |
|---|---|
| WBC | 4.5–11.0 ×10⁹/L |
| Neutrophils | 40–70% |
| Lymphocytes | 20–40% |
| Monocytes | 2–8% |
| Eosinophils | 1–4% |
| Basophils | 0–1% |
The differential — the breakdown by type — often says more than the total. A raised neutrophil count and a raised lymphocyte count point in different directions, and a total that is normal can conceal a shifted differential.
Red cells — oxygen carrying
| Value | Male | Female |
|---|---|---|
| RBC | 4.5–5.9 ×10¹²/L | 4.1–5.1 ×10¹²/L |
| Haemoglobin | 13.5–17.5 g/dL | 12.0–15.5 g/dL |
| Haematocrit | 41–53% | 36–46% |
Haemoglobin is the number that matters most here; it is what anaemia is defined by. Red cell values are sex-specific and the difference is large enough that using the wrong column produces a wrong conclusion.
Red cell indices — the shape of the cells
MCV (mean corpuscular volume), typically 80–100 fL, describes average red cell size and is the single most useful line for narrowing down a cause of anaemia. Small cells, normal cells and large cells point toward different explanations.
MCH and MCHC describe haemoglobin content and concentration. RDW describes how variable the cell sizes are, and a raised RDW with a normal MCV can be an early signal that something is changing.
Platelets — clotting
Typically 150–450 ×10⁹/L. Low platelets affect clotting; high platelets can reflect inflammation, iron deficiency, or a bone marrow process.
MPV describes average platelet size and is reported alongside.
Why a flagged value is often not a problem
Reference ranges are constructed so that about 95% of healthy people fall inside them. That arithmetic means roughly one healthy person in twenty falls outside any given range by chance alone.
A CBC reports a dozen values. The probability that at least one comes back flagged in a perfectly healthy person is therefore substantial — which is why a single marginal flag, with no symptoms and no trend, is usually watched rather than investigated.
What changes a result without any illness
Dehydration concentrates the blood and raises haemoglobin, haematocrit and red cell count.
Recent exercise, stress, or a recent meal can move white cell counts.
Time of day, altitude, pregnancy and smoking all shift baselines. Smokers run higher haemoglobin; residents at altitude higher still.
Recent illness or a course of steroids moves the white cell differential for days to weeks afterwards.
Reading a result sensibly
The useful comparisons are against your own previous results and across lines. A haemoglobin that has fallen 2 g/dL while remaining inside the range is more informative than one that sits marginally below the lower limit and always has.
Bring the actual report to the appointment rather than a summary. The laboratory's own ranges, the units, and any comment from the reporting scientist are all on it, and all three matter to interpretation.
This page explains what the lines represent. It is not a substitute for having the result interpreted by the clinician who ordered it.