Serum creatinine

Serum Creatinine

Measuring serum creatinine is a useful and inexpensive method of evaluating renal dysfunction. Creatinine is a non-protein waste product of creatine phosphate metabolism by skeletal muscle tissue. Creatinine production is continuous and is proportional to muscle mass.

Creatinine is freely filtered and therefore the serum creatinine level depends on the Glomerular Filtration Rate (GFR). Renal dysfunction diminishes the ability to filter creatinine and the serum creatinine rises. If the serum creatinine level doubles, the GFR is considered to have been halved. A threefold increase is considered to reflect a 75% loss of kidney function.

Reference values for serum creatinine:

Adult males: 0.8 - 1.4 mg/dl: values are slightly higher in males due to larger muscle mass
Adult females: 0.6 - 1.1 mg/dl: creatinine clearance is increased in pregnancy, resulting in lower serum levels
Children: 0.2 - 1.0 mg/dl: slight increases with age because values are proportional to body mass
A panic value for creatinine is 10 mg/dl in nondialysis patients.

Increased serum creatinine levels are seen in:

Impaired renal function
Chronic nephritis
Urinary tract obstruction
Muscle diseases such as gigantism, acromegaly, and myasthenia gravis
Congestive heart failure
Shock

Decreased creatinine levels may be seen in: the elderly, persons with small stature, decreased muscle mass, or inadequate dietary protein. Muscle atrophy can also result in decreased serum creatinine level. If muscle atrophy is suspected, assessment of serum creatine, an important enzyme necessary for normal muscle function, is done.

The only important pathological condition that causes a significant increase in the serum creatinine level is damage to a large number of nephrons. Unlike the BUN, the serum creatinine level is not affected by hepatic protein metabolism. Tests to measure serum creatinine, urine creatinine, and creatinine clearance are all used only to evaluate renal function. Only renal dysfunction changes the results. The serum creatinine level does not rise until at least half of the kidney's nephrons are destroyed or damaged. Because creatinine levels rise and fall more slowly than BUN levels, creatinine levels are often preferred to monitor renal function on a long-term basis.

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Measurement of serum creatinine is important because it is a inexpensive and reliable indicator of renal function.

Serum creatinine and Blood Urea nitrogen (BUN) are often compared to evaluate renal function. While serum creatinine increases only with nephron damage, the BUN is affected by hydration, hepatic metabolism of protein and reduced GFR. The mean ratio of serum creatinine to the BUN should be approximately 1:10.

NEPHROTOXIC DRUG	REDUCE TOXICITY
ACE inhibitors	Caution in hypovolemia Avoid in bilateral renal artery stenosis
ACYCLOVIR	Avoid bolus dosing Use IV hydration Titrate dose to renal function
Aminoglycosides	Check levels Correct potassium levels Give only one daily dose Titrate to renal function
Amphotericin B	Titrate dose to renal function IV normal saline hydration Liposomal formulation less toxic
Cisplatin	IV normal saline hydration
Cyclosporin	Titrate dose to renal function Avoid erythromycin, verapamil, ketoconazole
Indinavir	Hydrate maintain high urine output
Interleukin-2	IV hydration Possible albumin infusion
Intravenous contrast	IV normal saline hydration Possible acetylcysteine
Lithium	Titrate dose to renal function Possible Amiloride

The kidney's properties of high blood flow, filtration, reabsorption and concentration allow nephrotoxins to accumulate. A patient's BUN and creatinine should always be checked before giving nephrotoxic drugs. If either level is above the normal range, the drug should be held and the patient's physician should be contacted.

The commonly prescribed aminoglycoside antibiotics have both potential nephrotoxic and ototoxic effects. The impaired hearing or dizziness that may result from ototoxity is more likely if the drug is continued when there is renal dysfunction. It is also important to keep the patient well hydrated when aminoglycoside antibiotics are given because they are excreted almost unchanged in the urine.