How to read your blood test report without panicking

Reports arrive on your phone before you have seen a doctor, and a single bold "H" beside a value can ruin a weekend. It is worth understanding what those flags do and do not mean.

What a reference range really is

A reference range is not a boundary between healthy and unhealthy. It is the range covering roughly the middle 95% of results from a healthy reference population. By construction, about one healthy person in twenty falls outside it for any given test. Run twenty tests on a perfectly well person and, on average, one will be flagged.

This is why a marginally abnormal value in someone who feels entirely well is usually not alarming, and why doctors look at the pattern across your results rather than hunting for flags.

Things that change your results without anything being wrong

• Not fasting when the test required it — this affects glucose and lipid results substantially
• Dehydration, which concentrates many values
• Hard exercise in the day or two before the test
• A recent infection, which shifts white cell counts and inflammatory markers
• The time of day, which matters for hormones such as cortisol and thyroid tests
• Some medicines and supplements, including biotin, which can interfere with certain assays

Why trends matter more than single numbers

One reading is a snapshot. Three readings over a year are a story. A cholesterol figure that has been climbing steadily while staying technically within range can matter more than a single value just outside it. Keep your old reports, or ask us for them — we hold your history.

Results worth calling us about promptly

Some findings genuinely should not wait for your next scheduled appointment. Contact us soon if your report shows a markedly abnormal blood sugar, a significant drop in haemoglobin, a substantially abnormal kidney or liver result, a very high or very low white cell count, or if the laboratory has flagged any value as critical. Our laboratory contacts the treating doctor directly when a result is critical, so you may well hear from us first.

The single most useful thing you can do

Do not interpret your report alone, and do not interpret it by searching for each value online — that reliably finds the most frightening explanation rather than the most likely one. Bring the report to a consultation. A physician reading it alongside your symptoms, your history, and your previous results will reach a very different conclusion from a search engine reading it in isolation.

If you have a report from us and no appointment, you can book a General Medicine consultation to go through it. Bring the printout or the digital copy, and any earlier reports you have.