Creatinine clearance test: what the result means

A creatinine clearance test compares the amount of creatinine in urine with the amount in blood, in order to calculate how much blood plasma the kidneys clear of this substance each minute. Creatinine forms in muscle as creatine phosphate is broken down and enters the blood at a fairly steady rate, and the kidneys remove it almost solely by filtering it in the glomeruli. Creatinine clearance is therefore considered one of the ways of indirectly assessing the glomerular filtration rate (GFR), that is, how efficiently the kidneys work overall. The test requires urine collected over a set period (most often 24 hours) together with a blood sample taken to measure creatinine. The result depends on muscle mass, age, sex and physical activity, so the same number can mean a different state of the kidneys in different people. In clinical practice today, calculated GFR formulas based on blood creatinine are also often used, but a direct clearance test remains useful in certain situations — for example when muscle mass is unusual, or when a more precise assessment is needed.

• When impaired kidney filtration is suspected — when a general blood creatinine test gives a borderline or unclear result
• To monitor known chronic kidney disease and how it progresses over time
• To assess how the kidneys are working after an acute condition (for example a major loss of fluids, a severe infection) or after surgery
• Before prescribing medicines whose dose depends on kidney function, or to monitor their effect on the kidneys
• When investigating troubling symptoms that may be linked to the kidneys — swelling, a change in how often you pass urine, tiredness

A raised creatinine clearance rarely gives cause for concern in itself — it most often reflects a large muscle mass, intense physical activity or a meat-rich diet, all of which increase the amount of creatinine in urine. This indicator is rarely assessed as a condition in its own right — a low or a falling result matters more.

• A large muscle mass, or intense physical activity before the test
• A meat-rich diet before the sample was collected
• Inaccurate collection of the 24-hour urine sample (too large a volume)

Usually considered alongside a raised result: whether the preparation was followed during the test (avoiding meat, accurate urine collection); whether the result matches the other kidney markers — blood creatinine, GFR; and when to repeat the test with the sample collected more accurately.

A reduced creatinine clearance shows that the kidneys clear less plasma of creatinine per minute than usual — this can mean impaired kidney filtration. Per MedlinePlus, the causes include kidney damage, reduced blood flow to the kidneys, or a loss of body fluids.

• Impaired kidney function, including kidney failure
• Reduced blood flow to the kidneys
• Glomerulonephritis (inflammation of the kidney glomeruli)
• A loss of body fluids (dehydration)
• A blockage of the outflow tract of the bladder
• Heart failure

Usually assessed alongside a reduced result:

• Further kidney markers — blood creatinine, GFR, urea — for the overall picture
• An assessment of fluid balance and blood pressure
• A possible underlying cause — a heart, urinary tract or kidney condition
• When to repeat the test to follow the change over time