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Creatinine Clearance

An anhydride of creatine, creatinine is formed and excreted in constant amounts by an irreversible reaction and functions solely as the main end product of creatine. Creatinine production is proportional to total muscle mass and is relatively unaffected by urine volume or normal physical activity or diet.

An excellent diagnostic indicator of renal function, the creatinine clearance test determines how efficiently the kidneys are clearing creatinine from the blood. The rate of clearance is expressed in terms of the volume of blood (in milliliters) that can be cleared of creatinine in 1 minute. Creatinine levels become abnormal when more than 50% of the nephrons have been damaged.

Purpose

• To assess renal function (primarily glomerular filtration).
• To monitor progression of renal insufficiency.

Patient preparation

• Explain to the patient that this test assesses kidney function.
• Inform him that he needn't restrict fluids but shouldn't eat an excessive amount of meat before the test.
• Advise him that he should avoid strenuous physical exercise during the collection period.
• Tell him the test requires a timed urine specimen and at least one blood sample.
• Tell him how the urine specimen will be collected. Also inform him who'll perform the venipuncture and when and that he may feel some discomfort from the needle puncture. Reassure him that collecting the blood sample takes less than 3 minutes.
• Explain that more than one venipuncture may be necessary.

Procedure and posttest care

• Collect a timed urine specimen at 2, 6, 12, or 24 hours in a bottle containing a preservative to prevent degradation of creatinine.
• Perform a venipuncture anytime during the collection period, and collect the sample in a 7-ml red-top tube.
• Apply warm soaks to ease discomfort if a hematoma develops at the venipuncture site.
• Resume administration of medications withheld before the test.
• Tell the patient that he may resume normal diet and activity.

Precautions

• Refrigerate the urine specimen or keep it on ice during the collection period.
• Send the specimen to the laboratory as soon as the collection is complete.

Reference values

Abnormal findings

Low creatinine clearance may result from reduced renal blood flow (associated with shock or renal artery obstruction), acute tubular necrosis, acute or chronic glomerulonephritis, advanced bilateral chronic pyelonephritis, advanced bilateral renal lesions (which may occur in polycystic kidney disease, renal tuberculosis, and cancer), nephrosclerosis, heart failure, or severe dehydration.

High creatinine clearance rates generally have little diagnostic significance.

Interfering factors

• Failure to observe restrictions, to collect all urine during the test period, to properly store the specimen, or to send the completed sample to the laboratory immediately.
• Amphotericin B, thiazide diuretics, furosemide, and aminoglycosides (possible decrease).