How a range is made
A reference range describes the results found in a large group of healthy people. It is usually set to include the middle 95 in 100 of them. That means, by design, about 5 in 100 healthy people fall outside the range on any single test.
Why ranges differ
- Laboratories use different equipment and methods, so each lab publishes its own ranges.
- Some ranges differ by age, or change in pregnancy.
- Food, exercise, medicines, the time of day and how the sample was handled can all move a result.
What a clinician looks at
- How far outside the range the value is, and whether it has changed over time.
- Other results from the same test and related tests.
- Symptoms, history, examination and current medicines.
A result inside the range doesn't always rule a problem out, and one outside it doesn't always mean there is one. That is why results are interpreted by the clinician who ordered them.
Sources
- How to Understand Your Lab Results — MedlinePlus — U.S. National Library of Medicine. Checked 2026-09-26.
Editorial update: 2026-09-26.
Medical disclaimer: this is a source-linked educational summary, not individual medical advice, diagnosis, prescription or interpretation of personal results. For severe or rapidly worsening symptoms, call 115.
Antopeloni — plain-language health education from public sources, part of the ZIBADIS network founded by Masoud Moghaddam.
