What Is a Normal Range on a Blood Test?
How reference ranges are built
A lab measures a test in a large group of healthy people and takes the middle 95% of results. The edges become the “low” and “high” limits. That is a statistical definition, not a measure of health. It also means that if a panel includes 20 independent tests, a healthy person has a good chance of having at least one flagged result by chance.
Why ranges differ
- Method and instrument. Different analysers give slightly different numbers.
- Age and sex. Hemoglobin, creatinine, ferritin and hormones differ between men and women, and between children, adults and older adults.
- Population. A lab compares you with the people it has measured.
- Pregnancy. Many tests, including thyroid and kidney markers, have their own ranges.
- Units. A range in mmol/L looks completely different from the same range in mg/dL. See unit conversion.
“Normal” versus “optimal” versus “target”
These are different ideas. Normal describes the usual spread in healthy people. Optimal describes a level linked to the lowest risk, as with LDL below 100 mg/dL. A target is a personal goal your clinician sets, such as an HbA1c near 7% for some people with diabetes. Your own best number may be inside or outside the printed range.
What to do with a result just outside the range
- Check the unit and the range on the report.
- Look at your previous results. Is it new, or has it always been there?
- Consider recent causes: fasting, exercise, illness, new medicine.
- Ask your clinician whether to repeat it, and when.
You can look up many individual tests in our blood test library.
Frequently asked questions
Does “out of range” mean I am sick?
Not necessarily. About 1 in 20 healthy people fall outside the range on any single test, so context, trends and symptoms matter.
Why is my lab’s range different from one online?
Ranges depend on the lab’s method and population. Use the range printed on your own report.
What is the difference between normal and optimal?
Normal is the usual spread in healthy people. Optimal is a level linked to the lowest risk, and a target is a personal goal set with your clinician.