Medical tests and investigations explained

How to read a test result

Four things worth knowing before you look at a number

Most anxiety about test results comes from a handful of misunderstandings about what a test can and cannot tell you. None of these replace the explanation from the clinician who requested the test — they know why it was ordered, which is the part that gives a number meaning.

1. A "normal range" is a description of most healthy people, not a boundary of health. Reference ranges are set so that roughly 95% of healthy people fall inside them — which means about one healthy person in twenty sits outside a given range while being perfectly well. Ranges also differ between laboratories, so the same blood sample can be flagged at one hospital and not another. Compare a result against the range printed on your own report, never one from the internet.
2. A slightly abnormal result is usually a prompt, not a diagnosis. Clinicians read results as a pattern alongside your symptoms, examination and history. One marginal value in isolation is often repeated rather than acted on, because natural variation, a recent meal, exercise, dehydration or the time of day can all move it.
3. How useful a test is depends on how likely the condition was to begin with. This is why screening tests throw up false alarms: when a condition is rare, even a very accurate test produces more false positives than true ones. It is also why tests are not ordered "just to check everything" — an unfocused test is more likely to mislead than inform.
4. Some tests rule out; others rule in. A very sensitive test is good at excluding a condition when it comes back negative. A very specific test is good at confirming one when it comes back positive. Few tests do both well, which is why one test often leads to another.