The glomerular filtration rate, or GFR, for non-African Americans is typically lower because African Americans have, on average, greater muscle mass and thus higher serum creatinine levels, explains Cleveland Clinic. Serum creatinine is one among several markers that clinicians use to determine the glomerular filtration rate and measure kidney function. Apart from ethnicity, serum creatinine levels also depend on gender, aging and diet, among other factors.
Factors that lower aggregate muscle mass, such as amputations, malnutrition, aging and muscle atrophy, also attenuate levels of creatinine in the blood, reports Cleveland Clinic. Because women tend to have less muscle mass than men, their serum creatinine levels are typically lower. Ingesting cooked meat elevates blood creatinine concentration, while vegetarian diets lower it. Cimetidine, probenecid, potassium-sparing diuretics and other medications reduce creatinine secretion, leading to higher levels of the substance in the blood. Ketoacids, glucose, ascorbic acid and some forms of cephalosporins also boost levels of serum creatinine.
Muscle metabolism leads to the synthesis of creatinine from creatine phosphate and creatine, notes MedicineNet. The substance, a waste product, is carried by the blood to the kidneys, where the glomeruli filter it out into the urine. Because of this link between creatinine and the kidneys, clinicians use blood creatinine concentration as a proxy for kidney function. However, a better gauge of kidney function is the glomerular filtration rate, which measures the amount of creatinine that kidneys clear from the body in a given period.