eGFR

Many labs print '>90' or '>60' rather than an exact number; it falls gradually with age in the general population, so older adults commonly sit below 90 without any kidney illness.

The eGFR is not something measured directly. It is a calculation, made by the laboratory from the creatinine level together with the person's age and sex, that estimates how many millilitres of blood the kidneys filter each minute, scaled to an average body size. A higher number means more filtering.

It is the standard way doctors describe overall kidney function in a single figure, and it is printed automatically whenever creatinine is measured. It is used to follow kidney function over time and to check that certain medicines are being taken at an amount that suits the kidneys.

• Pregnancy, because the kidneys filter more
• Low muscle mass, which lowers creatinine and so raises the calculated eGFR
• Recovery of the kidneys after a short-term illness
• A meal or fluid intake that lowers creatinine on the day

• Long-term kidney disease, most often linked to diabetes or high blood pressure
• Dehydration or a short-term illness on the day
• Older age, as part of normal ageing
• Large muscle mass or creatine supplements, which raise creatinine
• Some medicines that change how creatinine is handled by the kidneys

• Because it is calculated from creatinine, anything that nudges creatinine (a big meat meal, hard exercise, dehydration) nudges the eGFR the other way
• The formula assumes average muscle mass, so it reads low in very muscular people and high in people with little muscle
• Labs cap the printed value at '>90' or '>60', so the number cannot be tracked precisely above that cut-off

Calculated from the same sample as creatinine; no extra sample is needed. Doctors look at the trend over months rather than one figure, and repeat it when a result is unexpected.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.