How we work with reference intervals

What is a reference interval?

When a laboratory establishes a reference interval, it measures a large number of healthy people and describes where their values land. The convention, followed among others by the Nordic Reference Interval Project NORIP, is that the interval spans from the 2.5th to the 97.5th percentile. That means it covers the middle 95 percent of the healthy reference group, and that 5 percent of perfectly healthy people fall outside it by definition.

This is worth pausing on. A reference interval is a descriptive measure, not a line between healthy and sick. A value just outside can be entirely normal for you, and a value inside does not rule out that something is drifting the wrong way. That is why results are always read in context: age, sex, symptoms and how the value has moved over time.

Why do different laboratories have different intervals?

Two laboratories can measure the same marker with different instruments, different reagents and against different reference groups. Their intervals then differ too, without either being wrong.

To reduce the differences, NORIP was carried out: 102 laboratories in the five Nordic countries collected samples from around 3000 healthy adults aged 18 and over, and produced common recommended intervals for 25 frequently used analyses. Many Swedish laboratories base their intervals on that work. But not every analysis is covered by NORIP, and for hormones, for example, laboratories set intervals based on their method. One university hospital laboratory lists 0.3 to 4.2 mE per liter for TSH in adults, while another laboratory may publish a somewhat different interval for the same marker.

The takeaway when reading a report: the value should always be compared against the interval from the exact laboratory that ran the analysis.

Where do our intervals come from?

We do not invent our own limits. The intervals and interpretation bands shown in Enkel Health come from published clinical sources, mainly of three kinds:

The analyzing laboratory's own reference intervals, which always accompany the analysis and are always shown in your report. The openly published sampling directories of university hospitals, where every analysis has documented intervals by age and sex. Nordic and international reference projects such as NORIP, plus guidelines from medical specialty societies where they exist.

Every limit in our system carries a record stating where it came from. That lets us show the source, review it and replace it when the source is updated. If no solid source exists for a limit, we show no limit, just your value against the laboratory's interval.

How are intervals reviewed before they are shown?

Described honestly, it works like this. Every marker in our library goes through a clinical review in which a licensed doctor checks that the source is sound, that the interval matches the source, and that the wording around the value holds up medically. Interpretation bands, the colored zones that help you read where in the interval you sit, are shown only once that review is complete and approved.

This review is ongoing as part of our launch work. No bands go live before they are clinically approved, and markers that have not yet passed review are shown without bands, with only the laboratory's interval. We choose that order deliberately: better an honest report with fewer layers than a polished zone map without backing.

What do you see in your report?

Three things, always in this order.

Your measured value, exactly as the laboratory reported it. The analyzing laboratory's reference interval, since that is the comparison that belongs to the method your sample was analyzed with. And where clinically approved interpretation bands exist, a plain-language explanation of what the position usually means and what may be sensible to do next.

If a published source and the laboratory's interval differ, the laboratory's interval is what governs the reading of your value, and it is what you see. Our explanations are educational support to help you understand the report. They never replace a doctor's assessment, and for off values healthcare is always the next step.

What do we do when the sources fall short?

Sometimes the literature scatters. Different sources can state different limits for the same marker, some markers lack established intervals for certain groups, and for a few debated values there are no generally accepted optimal levels at all.

Our position is simple. When the sources do not hold, we show no band, and we would rather write that the evidence is uncertain than pretend to a precision that does not exist. We flag internally which markers have weaker sourcing, and we update limits when the sources do. You will therefore notice that some values in Enkel Health carry richer explanations than others. It reflects how strong the evidence is, marker by marker.

This article is general health information, not medical advice and not an assessment of your values. If you have symptoms or concerns, contact your healthcare provider.