Creatinine is a byproduct in the breakdown of muscle creatine phosphate resulting from energy metabolism. It is produced at a constant rate depending on the muscle mass of the person and is removed from the body by the kidneys. Production of creatinine is constant as long as muscle mass remains constant. A disorder of kidney function reduces excretion of creatinine, resulting in increased blood creatinine levels. Thus, creatinine levels give an approximation of the glomerular filtration rate (GFR). An estimated GFR (eGFR) can be calculated using the Modification of 
Diet in Renal Disease study equation, which requires a serum creatinine result, gender, age, and race.

Many laboratories are reporting the eGFR with the creatinine result.

This test diagnoses impaired kidney function. It is a more specific and sensitive indicator of kidney disease than BUN, although in CKD, both BUN and creatinine are ordered to evaluate kidney problems because the BUN-to-creatinine ratio provides more information.

Adult men: 0.9–1.3 mg/dL or 80–115 μmol/L

Adult women: 0.6–1.1 mg/dL or 53–97 μmol/L

Children (3–18 years): 0.5–1.0 mg/dL or 44–88 μmol/L

Young children (0–3 years): 0.3–0.7 mg/dL or 27–62 μmol/L

BUN-to-creatinine ratio: 10:1–20:1

1. Obtain a 5-mL venous blood sample. Serum is preferred, but heparinized blood can be used. Label the specimen with the patient’s name, date and time of collection, and test(s) ordered. Place the specimen in a biohazard bag.

2. Observe standard precautions.

1. Increased blood creatinine levels occur in the following conditions:

   1. Impaired kidney function

   2. Chronic nephritis

   3. Obstruction of urinary tract

   4. Muscle disease

      1. Gigantism

      2. Acromegaly

      3. Myasthenia gravis

      4. Muscular dystrophy

      5. Poliomyelitis

   5. Heart failure

   6. Shock

   7. Dehydration

   8. Rhabdomyolysis (skeletal muscle tissue breakdown)

   9. Hyperthyroidism

2. Decreased creatinine levels occur in the following conditions:

   1. Small stature

   2. Decreased muscle mass

   3. Advanced and severe liver disease

   4. Inadequate dietary protein

   5. Pregnancy (0.4–0.6 mg/dL or 36–53 μmol/L is normal; >0.8 mg/dL or >71 μmol/L is abnormal and should be noted)

3. Increased ratio (>20:1) with normal creatinine occurs in the following conditions:

   1. Increased BUN (prerenal azotemia), heart failure, salt depletion, dehydration

   2. Catabolic states with tissue breakdown

   3. GI hemorrhage

   4. Impaired kidney function plus excess protein intake, production, or tissue breakdown

4. Increased ratio (>20:1) with elevated creatinine occurs in the following conditions:

   1. Obstruction of urinary tract

   2. Prerenal azotemia with kidney disease

5. Decreased ratio (<10:1) with decreased BUN occurs in the following conditions:

   1. Acute tubular necrosis

   2. Decreased urea synthesis as in severe liver disease or starvation

   3. Repeated dialysis

   4. SIADH

   5. Pregnancy

6. Decreased ratio (<10:1) with increased creatinine occurs in the following conditions:

   1. Phenacemide therapy (accelerates conversion of creatine to creatinine)

   2. Rhabdomyolysis (releases muscle creatinine)

   3. Muscular patients who develop kidney disease

Pretest Patient Care

1. Explain test purpose and procedure.

2. Assess diet for meat and protein intake.

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

1. High levels of ascorbic acid and cephalosporin antibiotic drugs can cause a falsely increased creatinine level; these agents also interfere with the BUN-to-creatinine ratio.

2. Drugs that influence kidney function plus other medications can cause a change in the blood creatinine level (see Appendix E).

3. A diet high in meat can cause increased creatinine levels.

4. Creatinine is falsely decreased by bilirubin, glucose, histidine, and quinidine compounds.

5. Ketoacidosis may increase serum creatinine substantially.