Reference Range
The band covering most healthy people — deliberately built so that one in twenty healthy people falls outside it.
What it means
The range within which most healthy people's results fall — conventionally set to include the middle 95%.
Which means, by construction, 5% of entirely healthy people fall outside it. One in twenty. That is not an error in the test; it is how the range was defined.
The consequence matters: on a panel of twenty tests, the odds of at least one flagged result in a completely healthy person are better than even. This is the single biggest reason broad health screens generate anxiety.
Ranges also differ between laboratories, and shift with age, sex, ethnicity and pregnancy — so comparing a result against a range from a different lab is not valid.
In practice
Printed beside every result you are ever given, usually with an H or L flag, or an asterisk, marking anything outside it.
Those flags are generated automatically by the analyser, not by a person deciding the result is concerning.
Why it matters
Do not read a flagged result as a diagnosis. Ask two questions: how far outside is it, and does it fit anything you are actually experiencing?
A result just over the line, in someone who feels well, is usually nothing. A result well outside, or one that fits your symptoms, is worth pursuing.
Trends beat single values. The same test six months apart tells you far more than one number against a population range.
Clinically reviewed by Dr Mohammad Zubair Khan, GMC 7563469
Last reviewed
August 30, 2026