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Introduction ⬇

Creatinine is a substance that, in health, is easily excreted by the kidney. It is the byproduct of muscle energy metabolism and is produced at a constant rate according to the muscle mass of the individual. Endogenous creatinine production is constant as long as the muscle mass remains constant. Because all creatinine filtered by the kidneys in a given time interval is excreted into the urine, creatinine levels are equivalent to the glomerular filtration rate (GFR). Disorders of kidney function prevent maximum excretion of creatinine. The creatinine clearance test is part of most batteries of quantitative urine tests. Creatinine clearance is measured together with other urinary components in order to interpret the overall excretion rate of the various urinary components.

The creatinine clearance test is a specific measurement of kidney function, primarily glomerular filtration. It measures the rate at which the kidneys clear creatinine from the blood. In a broad sense, clearance of a substance may be defined as the imaginary volume (in milliliters) of plasma from which the substance would have to be completely extracted in order for the kidney to excrete that amount in 1 minute. In addition to estimating the GFR, this test is used to evaluate renal function in patients.

Because the excretion of creatinine in a given person is relatively constant, the 24-hour urine creatinine level is used as a check on the completeness of a 24-hour urine collection. It is of no help in the evaluation of kidney function unless it is done as part of a creatinine clearance test.

Normal Findings ⬆ ⬇

Urine creatinine, men: 14–26 mg/kg/24 hr or 117–230 μmol/kg/d

Urine creatinine, women: 11–20 mg/kg/24 hr or 97–177 μmol/kg/d

Blood (serum) creatinine: 0.6–1.2 mg/dL or 53–106 μmol/L for males (Table 3.11)

0.4–1.0 mg/dL or 35–88 μmol/L for females

Estimated glomerular filtration rate (eGFR) is calculated using the serum creatinine results while accounting for age, sex, and race (see Chapter 6). eGFR is a measure of kidney function.

eGFR ≥90 mL/min/1.73 m2

Clinical Alert

eGFR 60 mL/min/1.73 m2 indicates kidney disease; 15 mL/min/1.73 m2 indicates end-stage kidney disease. Specific GFRs assist with classifying stages of kidney disease

Procedure ⬆ ⬇

  1. Properly label a 12-hour or 24-hour urine container with the patient’s name, date and time of collection, and test(s) ordered.

  2. Refrigerate or ice the specimen.

  3. Follow general instructions for 24-hour urine collection (see Long-Term, Timed Urine Specimen [2-Hour, 24-Hour]).

  4. Record exact starting and ending times on the specimen container and in the patient’s healthcare record.

  5. Send the entire specimen to the laboratory.

  6. For serum creatinine, obtain a 5-mL venous blood sample in a red-topped or serum separator tube.

  7. Record the patient’s height and weight on the container and in the patient’s healthcare record. Creatinine clearance values are based on the body surface area, and these values are needed to calculate the surface area.

  8. Ensure that the patient is adequately hydrated throughout the test to provide proper urine flow.

Clinical Implications ⬆ ⬇

  1. Decreased creatinine clearance is found in any condition that decreases kidney blood flow:

    1. Impaired kidney function, intrinsic kidney disease, glomerulonephritis, pyelonephritis, nephrotic syndrome, acute tubular dysfunction, amyloidosis, interstitial nephritis

    2. Shock, dehydration

    3. Hemorrhage

    4. Chronic obstructive lung disease

    5. Heart failure

  2. Increased creatinine clearance is found in:

    1. State of high cardiac output

    2. Pregnancy

    3. Burns

    4. Carbon monoxide poisoning

  3. Increased urine creatinine is found in:

    1. Acromegaly

    2. Gigantism

    3. DM

    4. Hypothyroidism

  4. Decreased urine creatinine is found in:

    1. Hyperthyroidism

    2. Anemia

    3. Muscular dystrophy

    4. Polymyositis, neurogenic atrophy

    5. Inflammatory muscle disease

    6. Advanced kidney disease, renal stenosis

    7. Leukemia

Interventions ⬆ ⬇

Pretest Patient Care

  1. Explain purpose of test, procedure for urine collection, and interfering factors. Written instructions can be helpful.

  2. Encourage fluids for good hydration. Large urine volumes ensure optimal test results. Advise patient to avoid tea and coffee (diuretics).

  3. Allow food but advise patient to avoid eating large amounts of meat. Check with healthcare provider.

  4. Avoid vigorous exercise during the test.

  5. Obtain healthcare provider’s approval to discontinue medications affecting the results before specimen collection, especially ACTH, cortisone, or thyroxine.

  6. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

Posttest Patient Care

  1. Patient may resume normal food, fluids, and activity.

  2. Review test results; report and record findings. Modify the nursing care plan as needed. Monitor appropriately.

  3. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.

Interfering Factors ⬆

  1. Exercise may increase creatinine clearance and urine creatinine.

  2. Pregnancy substantially increases creatinine clearance.

  3. Many drugs decrease creatinine clearance (see Appendix E).

  4. The creatinine clearance overestimates the GFR when there is severe renal impairment. The serum creatinine is more indicative of the GFR in this situation.

  5. A diet high in meat may elevate the urine creatinine concentration.

  6. Proteinuria and advanced kidney failure make creatinine clearance an unreliable method for determining GFR.

Clinical Alert

Determination of urine creatinine is of little value for evaluating kidney function unless it is done as part of a creatinine clearance test