Why two labs give you different ranges

People often compare a private result against an NHS range found online, or compare this year's result at one provider with last year's at another. Both comparisons can mislead, because reference ranges are specific to the assay that produced the number.

Different analysers, different ranges

Laboratories use analysers from different manufacturers, and for many hormones the platforms are not directly interchangeable. Each derives its own reference range from its own reference population on its own equipment. A testosterone of 11.5 nmol/L may sit inside one laboratory's range and just outside another's, with neither being wrong.

Units differ too

UK laboratories report testosterone in nmol/L; US laboratories use ng/dL. The conversion is roughly ng/dL ÷ 28.8 = nmol/L. A US result of 350 ng/dL is about 12.1 nmol/L — a number that reads alarmingly low against a UK range if the units are not converted first.

Ranges are population-specific

Some ranges are age-banded and some are not, which is why an older man may be told his testosterone is normal for his age against a range that has already accounted for decline. Female ranges depend on cycle phase and menopausal status, so a report that does not know where you are in your cycle can only offer a broad range.

Comparing across providers

• Stay with one provider when tracking a marker over time, if you can.
• If you must compare across labs, compare position within each range rather than absolute numbers.
• Keep the full report, not just the number — the range and units are the context that makes it interpretable.