Reading a complete blood count (CBC)

What a CBC measures

A complete blood count is one of the most common blood tests. From one sample it reports the main cell families in blood: red blood cells, which carry oxygen; white blood cells, which take part in immune defence; and platelets, which help blood clot. It also reports hemoglobin (the oxygen-carrying protein), hematocrit (the share of blood volume made up by red cells) and red-cell indices that describe cell size and hemoglobin content.

Many versions include a white-cell differential, which breaks the white-cell count into its subtypes, such as neutrophils and lymphocytes.

What the numbers are compared against

Each result is reported next to a reference interval: the range seen in most healthy people in the laboratory's population. A result outside the interval is a signal to interpret, not a diagnosis. Intervals differ between laboratories, ages and sexes, and a value slightly outside the range can still be normal for a particular person.

Clinicians read a CBC as a pattern, not as single numbers. For example, a low hemoglobin with small red cells points toward different causes than a low hemoglobin with large red cells, and the rest of the count plus the person's history decide what is checked next.

What it cannot tell you

A CBC does not diagnose a specific illness on its own. Many common conditions, from iron deficiency to infection to dehydration, move the same numbers in overlapping ways, and serious disease can occasionally coexist with a normal count. It also says nothing direct about organ function, cholesterol, blood sugar or most hormones, which need other tests.

Questions worth asking

Useful questions for the clinician who ordered the test: which part of the count mattered for the decision at hand, whether the result changed anything, and whether a repeat sample is planned. For education on how reference intervals and test accuracy work in general, this site's explainers on reference intervals and on sensitivity and specificity go deeper.