Reference ranges versus optimal values
Every value on your result comes with a range. Fall inside it and nothing is flagged. Fall outside and a letter appears next to it. What that range actually is rarely gets explained.
How a reference range is built
A laboratory measures a marker across a large group treated as the reference population. It then cuts off the outermost 2.5 percent at each end. What remains, the middle 95 percent, is the reference range.
Read that again, because something important is in it. The range describes how the value is distributed in a population. It does not describe the value at which someone feels well, nor the value at which future risk is lowest. It is a statistical boundary, not a health target.
Three consequences
By definition 5 percent fall outside
That is how the range was made. Out of every hundred healthy people, about five get a value with a letter next to it. Measure forty markers and the odds are good that one of them stands out for no reason at all.
The range depends on who was measured
If the reference population is a cross-section of the general population, some of it is not in optimal health. For lipids, if much of the population sits higher than desirable, the ceiling moves up with it. You can sit inside the range and still be above where you want to be.
Ranges differ per laboratory
Different equipment, method and reference population. Last year's result from another lab is therefore not always directly comparable to this one.
What a target range is instead
A target range comes not from population spread but from outcome research: at what value is risk or symptom burden lowest. For some markers that is well established, for others it is an estimate experts disagree on. That is why our report always shows both, with a note on how firm the distinction is.
| Reference range | Target range | |
|---|---|---|
| Where it comes from | The spread in a population | Outcome research |
| What outside means | Statistically unusual | Further from where you want to be |
| Differs per lab | Yes | No, though experts differ |
| Useful for | Spotting striking abnormalities | Steering over the long term |
Where the distinction matters most
- Ferritin. Many labs use a floor that is low compared to what is considered comfortable in practice. See the ferritin article.
- Vitamin D. Just above the floor is not the same as a good store. See vitamin D.
- Lipids and ApoB. Here reference and target sit furthest apart. See ApoB.
- Thyroid. TSH at the top of the range reads differently from TSH in the middle. See thyroid.
The trap on both sides
One trap is reading a normal result as proof that everything is fine. The other is treating every value not exactly on target as a problem. Neither holds. A result is a fact about one moment, and the direction it moves says more than the number itself.
That is why a second measurement is almost always more valuable than the first. See the check-up page.
Frequently asked questions
What exactly is a reference range?
The middle 95 percent of values in a reference population.
The outermost 2.5 percent at each end is cut off. It is a description of population spread, not a health target.
Why do reference ranges differ per laboratory?
Different equipment, methods and reference populations.
A result from another lab is therefore not always directly comparable. Prefer comparing measurements from the same laboratory.
Does outside the range mean something is wrong?
Not automatically.
About five in a hundred healthy people fall outside, because that is how the range was built. Measure forty markers and one often stands out meaninglessly.
Are optimal values scientifically established?
Well for some markers, less so for others.
For lipids and ApoB there is substantial outcome research. For other markers a target is more of an estimate experts disagree on. The report says which is which.
Why are there two columns on my result?
To make the distinction visible.
The lab reference shows whether you stand out relative to the population. The target range shows where you would want to be long term. They say different things.
Note: a blood test gives you insight and direction. It does not prove or rule out a condition on its own. Results are meant for prevention and for tracking change over time, not as a diagnosis. For symptoms or values outside the reference range, see your GP.