Blood and urine samples can both be used to test levels of kidney function. The kidney filtration rate can be estimated by measuring serum creatinine, a human waste product. The level of blood urea nitrogen (BUN) can also be tested to indicate how well the kidneys are functioning, although there are many factors that can affect the outcome of this particular test.
Perhaps the most effective way to test kidney function is to measure the glomerular filtration. There are various techniques that can be used to do this. The most accurate method is to measure creatinine, a breakdown product of muscle metabolism. The measurement of the creatinine content of a blood sample and a timed (e.g. 24 hour) urine collection allows for the accurate determination of the glomerular filtration rate.
As the glomerular filtration rate falls, the creatinine levels in the blood rise. With a fall in glomerular filtration rate by 50%, the blood creatinine needs to double in order for the body to excrete the normal amount of 1.4 grams per day. A further drop in glomerular filtration rate to 25% of the original level, causes the creatinine level to double again. Consequently, the blood creatinine level is a very sensitive index of kidney failure, or of changes in the level of kidney function, once the glomerular filtration has fallen significantly. Minor reductions below normal in glomerular filtration rate do not necessarily raise the creatinine level above normal, and can only be diagnosed by directly measuring the glomerular filtration rate.
In addition to the use of creatinine, there are nuclear medicine scanning techniques that can give an approximate assessment of glomerular filtration in each kidney. These techniques include x-ray, intravenous urography, computed tomography (CT) scan, and magnetic resonance imaging (MRI).