Creatinine clearance: measuring how well kidneys filter

Each kidney contains about a million tiny filters, the glomeruli. The glomerular filtration rate (GFR) is the volume of blood these filters clear per minute, and it is the best overall index of kidney function. Measuring true GFR directly is complex, so doctors use creatinine, a waste product made steadily by muscle and removed by the kidneys, as a practical stand-in.

Clearance is the amount cleared per unit time: if the kidneys remove creatinine well, a given quantity of it appears in urine for a modest blood level, and clearance is high. If filtration falls, blood creatinine rises and less appears in urine, so clearance falls.

A measured clearance combines a blood creatinine with a urine creatinine from a timed collection, usually 24 hours. Lab Tests Online UK explains that for a 24-hour collection you collect all your urine over a full day and are given instructions by your doctor or nurse. Missed samples spoil the result, so careful collection matters.

Because incomplete collections are common and the test is inconvenient, it has largely been replaced in routine care by calculated estimates. It is still used when an estimate is unreliable, for example in people with unusual muscle mass or diet, or when precise drug dosing is needed.

Lab Tests Online UK says an eGFR is also known as estimated creatinine clearance. It is calculated from the blood creatinine, age and sex, using the CKD-EPI equation recommended by NICE for most adults, and UK laboratories should report an eGFR alongside every creatinine result. It is expressed in millilitres per minute per 1.73 square metres of body surface.

The eGFR is most reliable in adults aged 18 to 74 and less accurate in pregnancy, malnutrition, or unusually high or low muscle mass. In people aged 75 or over or in those groups, a cystatin C test may be requested for a more accurate estimate, since cystatin C is less affected by muscle mass.

NICE guidance advises not eating meat or fish for 12 hours before the blood test, because protein can temporarily raise creatinine. Creatine supplements can also affect results, so mention them. Muscle mass matters: people with more muscle have higher creatinine, while older adults and those with muscle loss have lower creatinine for the same kidney function.

Heavy exercise, rhabdomyolysis, dehydration and some medicines also alter creatinine. A single result is a snapshot, and doctors look at the trend over time and at the urine albumin-to-creatinine ratio.

Neither measured clearance nor eGFR is accurate when kidney function is changing quickly, as in acute kidney injury, and eGFR should not be used to stage chronic kidney disease in that situation. A mildly low value that stays stable over years is less concerning than one that is falling. A normal or high eGFR can occasionally be seen in early diabetic kidney disease.

A reduced value is repeated, usually within a few weeks, alongside a urine albumin-to-creatinine ratio. If it is confirmed, the doctor looks for causes such as diabetes, high blood pressure, blockage or medicines, and refers to a kidney specialist depending on the stage and trend.