Doctors can assess kidney function by doing tests on blood and urine samples.
Creatinine, a waste product, is increased in the blood when kidney function is decreased by a large amount.
Creatinine clearance—a more accurate test than simply measuring the creatinine level in the blood—can be approximated from a blood sample using a formula that relates the creatinine level in the blood to a person’s age, weight, and sex. Determining creatinine clearance more precisely requires an accurately timed urine collection in conjunction with the blood creatinine determination.
Cystatin C, a protein in the blood, is also sometimes measured as an indicator of kidney function.
The level of urea nitrogen in the blood (BUN) can also indicate how well the kidneys are functioning, although many other factors can alter the BUN level.
(See also Evaluation of Kidney and Urinary Tract Disorders.)