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Creatinine Test

The creatinine test measures urine levels of creatinine, the chief metabolite of creatine. Produced in amounts proportional to total body muscle mass, creatinine is removed from the plasma primarily by glomerular filtration and is excreted in the urine. Because the body doesn't recycle it, creatinine has a relatively high, constant clearance rate, making it an efficient indicator of renal function. However, the creatinine clearance test, which measures both urine and plasma creatinine clearance, is a more precise index than this test. A standard method for determining urine creatinine levels is based on Jaffe's reaction, in which creatinine treated with an alkaline picrate solution yields a bright orange-red complex.

Purpose

• To help assess glomerular filtration.
• To check the accuracy of 24-hour urine collection, based on the relatively constant levels of creatinine excretion.

Patient preparation

• Explain to the patient that this test helps evaluate 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 usually requires urine collection over a 24-hour period and teach him the proper collection technique

Procedure and posttest care

• Collect the patient's urine over a 24­hour period. Use a specimen bottle that contains a preservative to prevent the degradation of creatinine.
• Resume administration of medications withheld during the test.
• Tell the patient he may resume nor­mal diet and activity.

Precautions

• Refrigerate the specimen or keep it on ice during the collection period.
• Once the collection is completed, send the specimen to the laboratory immediately.

Reference values

Normally, urine creatinine levels range from less than 0.8 to 1.8 g/24 hours in males and from 0.6 to 1.6 g/24 hours in females.

Abnormal findings

Decreased urine creatinine levels may result from impaired renal perfusion (associated with shock, for example) or from renal disease due to urinary tract obstruction. Chronic bilateral pyelonephritis, acute or chronic glomerulonephritis, and polycystic kidney disease may also depress creatinine levels. Increased levels 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 specimen to the laboratory immediately.
• Corticosteroids, gentamycin, tetracyclines, diuretics, and amphotericin B (possible decrease)