The creatinine test is used to diagnose impaired kidney function and to determine renal (kidney) damage.

A diet high in meat content can cause transient elevations of serum creatinine. Some drugs that may increase creatinine values include gentamicin, cimetidine, heavy-metal chemotherapeutic agents (e.g., cisplatin), and other drugs toxic to the kidneys, such as the cephalosporins.

The creatinine test is used to measure the amount of creatinine in the blood. Because creatinine is a nonprotein end-product of creatine phosphate, which is used in skeletal muscle contraction, the daily production of creatine, and the following product, creatinine, depends on muscle mass, which fluctuates very little.

Creatinine is excreted entirely by the kidneys, and therefore is directly related to renal function. When the kidneys are functioning normally, the serum creatinine level should remain constant and normal. Slight increases in creatine levels can appear after meals, especially after ingestion of large quantities of meat, and some diurnal variation may occur, with a low point at 7 A.M. and a peak at 7 P.M. Serious renal disorders, such as glomerulonephritis, pyelonephritis, and urinary obstruction, will cause abnormal elevations.

The creatinine level is interpreted in conjunction with another kidney function test called the Blood Urea Nitrogen (BUN). The serum creatinine level has much the same significance as the BUN but tends to rise later. Because of this, determinations of creatinine help to chronicle a disease process. Generally, a doubling of creatinine suggests a 50% reduction in kidney filtration rate.

The creatinine test requires a blood sample. It is recommended that the patient be fasting (nothing to eat or drink) for at least eight hours before the test. The physician may also require that ascorbic acid (vitamin C), barbiturates, and diuretics be withheld for 24 hours.

Risks for this test are minimal, but may include slight bleeding from the blood-drawing site, fainting or feeling lightheaded after venipuncture, or hematoma (blood accumulating under the puncture site).

Normal values can vary from laboratory to laboratory, but are generally in the following ranges:

• Adult female: 0.5–1.1 mg/dL
• Adult male: 0.6–1.2 mg/dL
• Adolescent: 0.5–1.0 mg/dL
• Child: 0.3–0.7 mg/dL
• Infant: 0.2–0.4 mg/dL
• Newborn: 0.3–1.2 mg/dL.

Note that variations between sources for serum creatinine normal ranges are greater than for other important tests. For example, due to the greater amount of muscle mass generally present, males normally demonstrate higher creatinine levels than females. Also, because the kidney filtration rate normally increases in pregnancy, serum creatinine should be slightly less during such periods. In older patients, creatinine is reduced because of decreased muscle mass. Similarly, other patients may have creatinine levels in which muscle abnormalities must be taken into consideration, such as long-term corticosteroid therapy, high thyroid (hyperthyroidism), muscular dystrophy, or paralysis.

Two to 4 mg/dL indicate the presence of impairment of renal function. Greater than 4 mg/dL indicates serious impairment in renal function.