Deriving normal ranges from abnormal patients

Doctors do millions of different tests on patients – bloods, imaging, exercise tolerance – and always define the range of normal values on healthy people. The normal haemoglobin concentration is based on healthy participants. Even in countries where malaria is common, and many people anaemic, the same normal ranges are used.

With two exceptions, renal function and fetal biometry. Until recently some kidney doctors used different estimated glomerular filtration rate (eGFR) ranges for people with black skins, who tend to have lower values, i.e. slightly worse renal function. And some obstetricians still use special “customised” fetal size charts, for short, underweight, or ethnic minority mothers, who tend to have smaller babies.

However the lower black eGFR values are not “normal”, they reflect more pathology, raised blood pressure etc. In 2021 NICE recognised that using different normal ranges for black people was racist. It deprived them of effective treatment, and NICE revised its guidelines to a single set of normal ranges for all adults (click here).

Most obstetricians also know that being short, under or over-weight, or having dark skin, is also associated with adverse outcomes, so using special charts for such women tends to normalise pathology. Their use is heightist, weightist and racist. The WHO, and the Intergrowth-21 group, have produced unified charts based on healthy pregnancies, which most obstetricians worldwide now use.

But the UK has been slow to adjust. About half of UK maternity units still use customised charts and the RCOG, in its Green Top guidelines, still endorses them (click here).

In the 1970s Archie Cochrane awarded obstetrics the “wooden spoon” as the last specialty to implement evidence-based medicine. We’ve done better in that department since.

But right now we are the last specialty using racist charts. Fortunately the Green Top guidelines are due for revision. It’s time to put this right.

Jim Thornton