How to read a blood test: the basics

A typical report lists each test by name, then your result, the unit it was measured in, and a reference range. A flag, such as H for high or L for low, appears when your result falls outside that range.

The reference range is the span of results seen in a large group of healthy people, and each laboratory sets its own. It is not a target or an ideal. Being inside the range does not always mean everything is fine, and being slightly outside it does not always mean something is wrong.

Check the unit before comparing results. The same test can be reported in different units, for example mg/dL or mmol/L, and ranges differ between laboratories, age groups and sexes. Compare like with like, ideally from the same laboratory.

One result is a snapshot. Many things can change a result on the day: whether you fasted, the time of day, how well hydrated you were, recent exercise, illness, stress, and some medicines and supplements. Trends across several tests are usually more informative than a single number.

Common tests include a full blood count (red cells, white cells and platelets), cholesterol and other blood fats, blood sugar and HbA1c, iron markers such as ferritin, vitamin D, and thyroid tests. Each answers a different question, and they are best read together rather than one at a time.

Do not diagnose yourself from a report, and do not start or stop treatment or supplements because of one result. Take the report to a doctor, bring earlier results for comparison, and ask what the numbers mean for you, whether a repeat test is needed, and what to do next.