Creatinine is the end product of creatine metabolism. Creatine, although synthesized largely in the liver, resides almost exclusively in skeletal muscle, where it reversibly combines with phosphate to form the energy storage compound phosphocreatine. This reaction (creatine + phosphate phosphocreatine) repeats as energy is released and regenerated, but in the process small amounts of creatine are irreversibly converted to creatinine, which serves no useful function and circulates only for transportation to the kidneys. The amount of creatinine generated in an individual is proportional to the mass of skeletal muscle present; level of muscular activity is not a critical determinant.

Daily generation of creatinine remains fairly constant unless crushing injury or degenerative diseases cause massive muscle damage. The kidneys excrete creatinine very efficiently. Levels of blood and urine flow affect creatinine excretion much less than they influence urea excretion because temporary alterations in renal blood flow and glomerular function can be compensated by increased tubular secretion of creatinine. Thus, serum creatinine is a more sensitive indicator of renal function than is urea nitrogen.²⁰

• Known or suspected impairment of renal function, including therapy with nephrotoxic drugs:
  • In the absence of disorders affecting muscle mass, elevated creatinine levels indicate decreased renal function.
  • Creatinine levels may be normal in situations in which a slow decline in renal function occurs simultaneously with a slow decline in muscle mass, as may occur in elderly individuals (in such situations, a 24-hour urine collection yields lower than normal excretion levels).
• Along with a urea nitrogen, to provide additional client information:
  • An elevated urea nitrogen with a normal creatinine usually indicates a nonrenal cause for the excessive urea.
  • The urea nitrogen rises more steeply than creatinine as renal function declines, and it falls more rapidly with dialysis.
  • With severe, permanent renal impairment, urea levels continue to climb, but creatinine values tend to plateau (at very high circulating creatinine levels, some is excreted through the gastrointestinal tract).
• Known or suspected disorder involving muscles, including crushing injury to muscles:
  • In the absence of renal disease, elevated serum creatinine levels are associated with trauma or disorders causing excessive muscle mass (gigantism, acromegaly).
  • Decreased levels are associated with muscular dystrophy.

Nursing Care Before the Procedure

Client preparation is the same as that for any test involving collection of a peripheral blood sample (see Appendix I).

• Some laboratories require an 8-hour fast before the test.

A venipuncture is performed and the sample collected in a red-topped tube. The sample should be sent promptly to the laboratory.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any study involving the collection of a peripheral blood sample.

• Complications and precautions: Increased levels should be assessed in relation to the urea nitrogen; notify the physician immediately if levels are grea-ter than 10 mg/dL unless the client is on dialysis.