Abnormal blood test results: should you worry, and what happens next?

Abnormal means outside a range, not ill

An abnormal blood test result means your value sits above or below the reference range your laboratory printed beside it. It is a comparison with healthy people measured on the same method, not a diagnosis.

A laboratory builds its range by testing healthy people and keeping the band that contains 19 in every 20 of their results. The one in twenty who land outside it are healthy too; they sit at the edges. A 2025 review in the Journal of the American Board of Family Medicine puts the consequence plainly: the number of results falling outside the normal range far exceeds the number that are clinically meaningful.

So the flag on your results is doing what it was built to do. It does not say you are ill, and the same review notes that minor variations labelled high or low cause patients unwarranted worry. If you are worried right now, that is the mechanism working on you, not evidence about your health.

Why healthy people get flagged

The arithmetic catches almost everyone with a long set of results. Each test leaves about one healthy person in twenty outside its range, and the chances add up: on a panel of 20 tests, the review calculates that about two healthy people in three will have at least one result outside it.

British data show how ordinary this is. In a study from Tayside in Scotland, about a quarter of adults had liver blood tests over ten years, and about a third of them had at least one abnormal value. At one Birmingham hospital, 30% of GP requests for liver tests in 2016 contained at least one abnormal result.

What your GP’s comment means

In England, results ordered by your GP are reviewed by a healthcare professional before they appear in the NHS App, usually within a few days, and many surgeries add a short comment. The words vary between practices. One NHS practice in Hull explains its comments like this:

The NHS says that if your results do not show anything, you may not need to do anything else. If you have heard nothing after a few weeks, contact the GP surgery or the specialist who ordered the test.

The four questions that sort it out

Read every flagged line through the same four questions, with your own figures in front of you.

The last question has a technical answer. Laboratories can calculate how large a change between two of your results must be before it exceeds the normal day-to-day variation of that test, an approach a 2020 review describes for interpreting serial results. If you have earlier results, it is worth asking whether the change you see is bigger than that.

What can cause an abnormal result

Many causes have nothing to do with illness. Preparation comes first: a fasting glucose taken after breakfast will read high. European recommendations on blood sampling treat patient preparation, fasting included, as part of a valid result, while NICE says a fasting sample is not needed for a cholesterol test. NICE also advises adults not to eat meat in the 12 hours before a kidney test that reports an eGFR.

Timing is next. TSH follows a daily rhythm and runs markedly lower in the afternoon and evening than early in the morning, enough that one laboratory group proposes time-adjusted cut-offs to avoid false alarms. Habits count too: healthy smokers show higher white cell counts and haemoglobin than non-smokers. Mention any new medicine or supplement to whoever reads your results.

And the laboratory itself: ranges differ between laboratories because they depend on the analyser, the reagents and the population served. One NHS trust prints a ferritin range of 30 to 337 µg/L for all adults, another 11 to 310 for women. Comparing your value with a range from another laboratory, or from a table online, is one of the easiest mistakes to make.

Should you worry? When to act sooner

Some situations deserve a prompt call rather than a wait for the next appointment: a value far outside its range, several related lines moving the same way, a sharp change from your previous result, or any abnormal result alongside symptoms that worry you.

For the most dangerous results, the system does not rely on you. Royal College of Pathologists guidance asks laboratories to phone critical results to the clinician who requested the test, usually within 2 hours. If your surgery has texted or called asking you to book an appointment about a result, that is a different situation, and it is not one to research: ring them.

Context is what turns a flag into a decision. In a study of 477,870 patients in England who saw their GP with abdominal pain or bloating, a single blood result said little on its own, but read alongside symptoms, age and sex it could change the picture: for women in their fifties with bloating, a raised platelet count lifted the chance of cancer from 1.6% to 8%. That is the judgement your GP makes, with information a flag does not have.

Many people now see results before anyone explains them. A 2026 review of 39 studies found that borderline values are what people find hardest to judge. If that is you, the four questions above are the most useful way to spend the wait.

What to do with an abnormal result

Do not start with a search engine on the test name. Start with the reason the test was ordered: if it was for tiredness, the lines that answer that question come first, and an unrelated flag can wait for your appointment.

Write down the four questions with your own figures and take them with you, along with anything that could have moved the result: a meal, the time of the test, a recent illness, exercise, smoking or a new medicine. For many mildly abnormal values, a sensible thing to ask is whether the test should simply be repeated under the same conditions.

Resist booking a wider private panel to settle it. A Cochrane review of general health checks found little or no effect on deaths, and every extra line brings another chance of a flag to explain.