Why your lab's reference range differs

Reference ranges are not universal

Every laboratory sets its own reference range for each test it runs. The range you see printed on your results report - usually as a bracketed interval next to your value - is specific to that laboratory, that particular testing machine, and the method (assay) they use.

This means the same blood sample, if sent to two different labs, could return slightly different numerical values and slightly different reference ranges. Both results could be accurate. They are simply produced using different equipment and calibrated against different populations.

Why ranges vary between labs

Several factors contribute to variation in reference ranges:

• Different assays. Labs use different commercial testing methods, each calibrated slightly differently. A TSH assay from one manufacturer may read a little higher or lower than another, even on the same blood sample.
• Population differences. A reference range is conventionally set to cover about 95% of a healthy population - which also means around 1 in 20 healthy people fall marginally outside it. If labs draw their populations from different areas or demographic groups, the resulting ranges can differ.
• Method updates. Laboratories occasionally switch to newer assay versions or instruments, which may shift the range - sometimes by enough to move a borderline result from inside to outside the range, or vice versa.

The most important rule: compare to YOUR report's range

When you look at a result, the number only makes sense in relation to the reference range printed on the same report. A TSH of 4.5 mIU/L might sit just inside one lab's range and just outside another's.

This is why comparing your results to ranges found on health websites, in books, or shared in patient communities can be misleading. The reference range that matters is the one on your actual report.

The lab results tool is designed around this principle - it asks you to enter the range from your report rather than applying a fixed universal range.

What it means when your range changes between results

If the reference range on your latest result looks different from a previous one, the most likely explanation is that your sample was processed at a different laboratory or that the lab updated its assay. It is not usually a sign that something has changed with your health.

However, it does mean that results from different labs cannot always be directly compared number for number. If you switch GP practice, hospital, or if your lab changes its methods, it is worth noting this when reviewing trends over time. Your clinician will take this into account when interpreting your results.

Adjusting your settings

If you track results in the lab results tool, you can update the reference range in settings whenever your report shows a different one. This keeps your visual comparisons accurate to the range that was in use at the time each test was taken.

Questions worth raising with your clinician

• Has the laboratory or assay changed since my last test?
• Should a result that sits just outside the reference range always be acted on, or does proximity to the range matter?
• If I am tested at different hospitals or labs, how do you account for range differences when comparing results?