To read blood test results, look at three things on each line: the result, the units and the reference range printed by your lab. An H, L, asterisk or arrow only means the number fell outside that range. It does not, on its own, mean you are ill: about 1 in 20 healthy people land just outside the range for any single test.
Whether it comes from an NHS hospital, a private clinic in the UK or a lab such as Quest or Labcorp in the US, almost every report is a table. Read each row from left to right: what was measured, how much was found, and what it should be compared with. A number without its unit and range tells you nothing: a glucose of 5.4 is normal in mmol/L and would be impossibly low in mg/dL.
These flags are added automatically by the lab system as soon as a number crosses the edge of the range. H or ↑ means above, L or ↓ means below, and an asterisk (*) or "A" means abnormal in either direction. Some labs also use HH or LL, or "critical", for values far outside the range; these are usually phoned through to your GP or doctor the same day.
A flag does not weigh how far outside the range you are. A cholesterol 0.1 above the limit and one twice the limit get exactly the same H. That is why it is worth looking at the number itself, not only at the symbol.
Reference ranges are usually built so that they cover the middle 95% of healthy people. By definition, 5% of healthy people fall outside the range for each test. If you have 20 or 30 tests on one report, it is quite likely that one or two will be flagged even if you are perfectly well.
Ordinary things also shift results: not fasting before glucose or triglycerides, a hard workout the day before (which raises CK and sometimes creatinine), dehydration, a recent cold, pregnancy, your menstrual cycle, or the time of day the sample was taken. A result slightly out of range is often simply repeated a few weeks later.
Ranges depend on the analyser, the method and the population the lab is based on, so they differ between hospitals and between countries. Always compare your result with the range printed on the same report, not with a figure found online. Units matter too: cholesterol of 5.2 mmol/L in the UK is the same as about 200 mg/dL in the US, and haemoglobin of 120 g/L is 12.0 g/dL.
If you change labs or move country, look at the trend in each lab's own range rather than comparing raw numbers.
Most results flagged on a routine test are discussed at a normal appointment. Contact your GP or doctor sooner, or seek urgent care (NHS 111, A&E, urgent care or the emergency room), if:
Written using public information from:
No need to decode the numbers yourself: take a photo of the report and the app explains “How to read blood test results” and the rest in plain words — what is normal and what deserves attention.
App StoreGoogle PlayWeb versionThat your value is above or below the range most healthy people have at that lab. It is a prompt to look closer, not a diagnosis. A small deviation in one test, with everything else normal and no symptoms, is often repeated rather than investigated.
Usually not. Ranges are set so that 5% of healthy people fall outside them for each test. Your doctor will look at how far out it is, at related tests and at your history before deciding whether anything needs doing.
H means higher than the reference range and L means lower. They are added automatically and do not show how big the difference is, so read the number and the range next to it.
Most UK labs report in SI units (mmol/L, µmol/L, g/L) and most US labs in conventional units (mg/dL, g/dL). The underlying value is the same; only the scale changes. Check the unit before comparing with anything you read.
You can understand what each test measures and how your number compares with the range. Deciding what it means for you, and whether anything should be done, needs your doctor, who knows your history and examination findings.
This material is for general information only. It is not a medical opinion and does not replace a consultation with a doctor. Reference ranges depend on the laboratory and the method — rely on the range printed on your own report. Your doctor makes the final interpretation.
Ponyatno