Muscle metabolism –> creatine –> creatinine (nonenzymatically converted). Creatinine production is constant, related to muscle mass (20-25 mg/kg males, 15-20 mg/kg females). Filtered but not reabsorbed in kidneys. Therefore, directly related to muscle mass, inversely related to glomerular filtration. More reliable than BUN because body muscle mass is fairly constant for a given individual. Normal values: 0.8-1.3 males, 0.6-1.0 females. Double serum creatinine = 50% reduction of GFR
Variables which affect creatinine levels
Cimetidine, large meat meals, ketoacidosis all elevate creatinine without changing GFR.
GFR changes with aging
GFR decreases 5% per decade after age 20, but muscle mass also declines, therefore serum levels remain normal although creatinine production decreases. Therefore, in elderly, small increase in creatinine = large change in GFR.
Ratio increases above 10:1 during low renal tubular flow states (decreased renal perfusion, urinary tract obstruction, etc). Also increases during increases in protein catabolism. BUN:creatinine ratios greater than 15:1 seen in volume depletion, CHF, cirrhosis, nephrotic syndrome, obstructive uropathies, and GI bleeding.
Most accurate study to evaluate renal function (GFR) (Urine creatinine x Urine flow rate)/(Serum creatinine). Usually calculated via 24 hour urine, but 2 hour tests are reasonably accurate in critically ill patients. Normal values: males 97-137, females 88-128. Mild renal impairment -> CCR 40-60, CCR 25-40 = moderate impairment and almost always causes symptoms.
Progressive renal disease enhances creatinine secretion in proximal tubules. Therefore, with progressively worsening renal disease, CCR progressively overestimates true GFR.
Perioperative assessment of renal function
Urine flow rate, specific gravity, and osmolality are all poor indicators of renal dysfunction because they are influenced by nonrenal factors.
BUN and creatinine are good screening tools, but poor tools to predict renal dysfunction because they are late warning signs of declining renal function.
Again, CCR is the best study to monitor renal function and predict potential ARF.