Reference ranges in blood testing: why normal is not the same for everyone

The reference range is the band next to almost every blood result. It shows where the values of about 95 percent of a healthy group fall. If you land just outside it, you belong to the rarer extremes, not automatically to the sick. In our experience this is the most misunderstood part of a result, which is a shame, because there is a lot of reassurance here (Sikaris 2014).

This article expands on the overview about your blood values, focusing on how reference ranges arise and why they differ.

What exactly is a reference range?

A reference range comes from measuring many healthy people and taking the middle band, usually the central part of about 95 percent of that group. The lowest and highest few percent fall outside by definition. So outside the range is not the same as abnormal in a medical sense (Ozarda 2016).

Why does normal differ per person?

Reference ranges are an average, and averages never apply to everyone exactly. Your sex, age and situation colour what is usual for you.

Take hemoglobin: the average is higher in men than in women. So the same result can be normal for one person and slightly low for another.

Why do labs differ from each other?

Two labs can measure the same substance slightly differently, with other equipment and methods. So each lab sets its own reference range. A result that lands just inside at one lab can be just outside at another. It means you best compare your values within the same lab.

What does this mean for your result?

In short: read your reference range as context, not a final verdict. A value just outside the range is often a reason to repeat, not to worry. Read more at an abnormal blood value. To measure your values without a referral and have them read in context, you can do a basic health checkup at Vitalcheck. Every result is reviewed by a BIG-registered doctor.

Frequently asked questions about reference ranges

Does outside the range mean I am ill?

Not necessarily. Because the range is a statistical average, part of healthy people fall just outside by definition. A mild deviation is often repeated and read in context. Discuss a clear deviation with your GP.

Why do my old and new results show different ranges?

Often the measurements were done at different labs, with other methods and so other reference ranges. Compare values within the same lab where you can.

Does the same reference range apply to men and women?

For many values it does not. Hemoglobin, ferritin and some hormones have different averages per sex. See also the overview about your blood values.

References

• Sikaris KA. Physiology and its importance for reference intervals. Clin Biochem Rev. 2014. PMID: 24659833.
• Ozarda Y. Reference intervals: current status, recent developments and future considerations. Biochem Med (Zagreb). 2016. PMID: 26981015.
• Thuisarts.nl. About blood testing.

Every blood test result includes a professional assessment from a BIG-registered doctor. For treatment decisions, discuss your results with your GP.