Renal function tests

A creatinine test assesses kidney function and is usually given with a BUN (the blood urea nitrogen test). It is part of a routine metabolic panel ordered as part of an overall check-up. It is also ordered to check on kidney function (for instance, if someone has diabetes) or to monitor a drug treatment for possible kidney toxicity.

The muscles produce creatine, a chemical that is made by the body and used to store and transfer energy. After muscles use creatine, it is broken down to creatinine, a waste product then excreted from the kidneys. Both creatine and creatinine are usually produced at a constant rate and the amount varies from person to person according to weight and muscle mass. For this reason, height and weight is sometimes asked of a person receiving a creatinine test. The test is generally not affected by exercise or diet. Recent muscle damage will affect the test result temporarily, as will taking a creatinine supplement.

A variance in creatinine level in a person over time may be an indication that there is a problem with the kidneys, but this level can also be affected by the use of certain drugs, including aminoglycosides and cephalosporins. In these cases, the creatinine concentration is increased, but is not an indication of kidney damage.

Some physicians order a creatinine and/or BUN test before certain procedures such as CT if a contrast agent is to be used, because the contrast agent may be harmful if there is already ongoing kidney damage or dysfunction. A small amount of urea nitrogen is present in the blood at a steady state and any disease, condition, or toxicity affecting the kidney or liver will usually affect the amount of urea present in blood.

A BUN/creatinine ratio is usually anywhere between 10:1 and 20:1. A higher ratio could indicate reduced blood flow to the kidneys (as in dehydration or congestive heart failure); gastrointestinal bleeding; or increased protein in the diet. A decreased ratio would indicate liver disease or malnutrition.

Combining the results of both a blood test and urine creatinine levels are used to calculate 'creatinine clearance', a measure of how effectively the kidneys are filtering small molecules from the blood. This test requires collecting all urine over a 24-hour period. If even one 'catch' is missed, the test results are invalid.

The creatinine clearance level is generally being replaced by a simpler and more accurate method of detecting changes in kidney status, using an estimated glomerular filtration rate (GFR or eGFR). Glomeruli are tiny filters in the blood that remove waste and prevent loss of proteins and blood cells. An eGFR rate is an indirect measure of the amount of blood that the kidneys filter per minute. A urine sample is not required.

The GFR is calculated using the serum creatinine concentration figure, adjusted for age, gender, and race. If a serum creatinine test has been ordered, the lab can also calculate the eGFR at no extra cost. If you have a creatinine level and want to calculate the eGFR, a simple and free calculator can be found online at the National Kidney Foundation (United States) website at www.kidney.org/professionals/tools.

If someone has normal eGFR or creatinine test results, but kidney dysfunction is suspected, a cystatin C test can be done. This test can monitor acute and chronic kidney disease and will detect deteriorating kidney function before it can be detected using an eGFR. Cystatin C is a cysteine proteinase inhibitor.

The utility of this test is still being explored in non-diabetic chronic kidney disease. One observational, multicentre that examined baseline blood samples of over 800 patients with stage 3 or 4 non-diabetic chronic kidney disease in the ‘Modification of diet in renal disease study’ concluded that serum cystatin C is at least as strongly associated with mortality as are GFR or creatinine concentration.¹ An advantage of the test is that it is not affected by gender, age, race, or muscle mass.

Creatinine, BUN, eGFR, and cystatin C are all obtained from a blood sample drawn from a vein in the arm.

Renal function tests provide information on how well the kidneys are working and are useful in diagnosing and monitoring dysfunctions caused by medication, HIV-associated nephropathy, or other cause kidney disease.