The kidney function number on your blood test, and what it does and does not mean

Estimated glomerular filtration rate appears on most routine blood panels, and a value below 60 generates a great deal of anxiety. Some of that anxiety is warranted and a good deal is not, and the difference is worth understanding.

What eGFR is

It is an estimate, calculated from serum creatinine along with age and sex, of how much blood the kidneys filter per minute. Creatinine is a waste product of muscle metabolism, cleared almost entirely by the kidneys, so it rises as filtration falls.

The reliance on muscle is the source of most of the test's limitations. A person with low muscle mass — which describes a great many older adults, and particularly frail ones — produces less creatinine, so their creatinine reads low and their eGFR reads high. Their kidney function may be considerably worse than the number suggests.

The reverse applies in muscular individuals, and creatinine also rises transiently with a large meat meal, with creatine supplementation, and with several drugs — trimethoprim and cimetidine among them — that block its tubular secretion without affecting actual filtration.

The staging, and the argument about it

Chronic kidney disease is staged by eGFR: stage 1 above 90 with other evidence of kidney damage, stage 2 from 60 to 89, stage 3a from 45 to 59, stage 3b from 30 to 44, stage 4 from 15 to 29, and stage 5 below 15.

The contested part is that eGFR declines with age in almost everyone — typically around 0.8 to 1 ml/min per year after forty. By eighty, a substantial share of people with no kidney disease at all have an eGFR below 60 and are therefore labelled with stage 3 chronic kidney disease.

Whether that represents disease or normal ageing has been argued in the nephrology literature for years. The practical resolution is that isolated mildly reduced eGFR in an older adult, stable over time and with no protein in the urine, carries a low risk of progression to kidney failure — most such people will die with it rather than of it.

What actually matters

Three things determine whether reduced kidney function is a concern.

The trajectory. A stable eGFR over years is reassuring. A falling one — more than about 5 ml/min per year — warrants investigation whatever the absolute value.

Albuminuria, as above.

The cause. Diabetes and hypertension account for the majority. Others include glomerulonephritis, polycystic kidney disease, obstruction — prostatic in men — recurrent infection, and drug-induced damage.

What to do about it

Blood pressure control is the single most effective intervention for slowing progression.

ACE inhibitors or ARBs where there is albuminuria, which reduce protein loss and slow decline. They typically cause a small rise in creatinine on starting — up to about 30 per cent is expected and acceptable, and stopping the drug in response to it is a common error.

SGLT2 inhibitors, which have become one of the more significant developments in nephrology. The DAPA-CKD and EMPA-KIDNEY trials showed slowed progression in chronic kidney disease with and without diabetes.

Glycaemic control in diabetes.

Avoiding nephrotoxins. NSAIDs are the main one and are freely available over the counter. Contrast agents, some antibiotics, and the combination of an ACE inhibitor or ARB with a diuretic and an NSAID — the triple whammy — which is a leading cause of acute kidney injury in older adults.

Medication dosing, which is where it bites

A great many drugs require dose adjustment in reduced kidney function, and failure to adjust is a common source of harm in older adults. Direct oral anticoagulants, metformin, several antibiotics, gabapentin and pregabalin, digoxin, opioids and allopurinol among them.

Two notes. For most drug dosing, absolute clearance rather than the body-surface-area-normalised eGFR is technically correct, which matters at extremes of body size. And for DOACs specifically, the licensing trials used Cockcroft-Gault creatinine clearance, not eGFR — a distinction that leads to inappropriate under-dosing in practice, leaving patients with the bleeding risk and less of the stroke protection.

Sick day rules

Worth knowing and rarely given. During an illness with vomiting, diarrhoea or fever — anything causing dehydration — certain drugs should be temporarily stopped to protect the kidneys: ACE inhibitors and ARBs, diuretics, metformin, NSAIDs and SGLT2 inhibitors.

Anyone with reduced kidney function on these drugs should have been given this instruction. Most have not. It is worth asking for it explicitly, in writing.