What a reference range is

Ranges are set so that about 95% of healthy people fall inside them. That means roughly 1 in 20 healthy people will be slightly outside the range for any single marker, and the more markers you test, the more likely it is that one will be flagged.

High and low flags

Your report highlights results above or below the range. Small differences often reflect hydration, recent exercise, a recent illness or the time of day. A large difference, or several related results out of range, is more likely to mean something.

Trends matter more than one result

One reading is a snapshot. Comparing results over time, ideally taken the same way and at the same time of day, shows the real trend. That’s why your GP may repeat a test before making any decisions.

Units can differ

The UK uses different units from some countries. For example, cholesterol is measured in mmol/L here but mg/dL in the US. Always compare your result with the range printed on the same report.

What to do next

Share any result outside the range with your GP, especially if you have symptoms. If you feel very unwell, don’t wait: call NHS 111, or 999 in an emergency.

Please note: this guide is general information and doesn’t replace advice from your GP. If you feel very unwell, call NHS 111, or 999 in an emergency.

Tests mentioned in this guide

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