The kidneys filter blood plasma, removing metabolic wastes and excrete them in urine.
When kidney function declines, waste products accumulate in the blood.
Their levels can be measured in blood tests to monitor kidney function.
Two substances are usually measured: creatinine, a waste product from the normal turnover of muscles;
and urea, the primary nitrogenous waste derived from metabolism of proteins. Blood levels of these
substances RISE as kidney function declines. Blood is filtered in functional units of the
kidney called the nephrons. Filtration occurs primarily in the glomerulus of the nephron.
The amount of filtrate produced per minute is called glomerular filtration rate, GFR. GFR is
an indicator of how well the blood is filtered by the kidneys. GFR is, however, difficult to
measure directly. In practice, it is usually calculated as a function of serum creatinine.
The formula takes into account the patient’s age, gender and race. A normal GFR is greater
than 90 mL per minute. A GFR below 60 is a sign of reduced kidney function. A GFR smaller than 15
represents end-stage renal disease, for which dialysis or a kidney transplant is required.
Urinalysis is a series of urine tests. Urinalysis includes visual, microscopic examination of urine
samples, as well as dipstick tests, which measure various substances in urine. The stick shows color
changes if certain substances are present or if their levels are above normal. Urinalysis
can help detect not only renal disorders but also urinary tract infections and other conditions such
as diabetes and liver damage. For kidney function, albumin is tested. Albumin is a blood protein that
is not normally passed into urine. Presence of albumin in urine is a sign of renal disease.
Urine albumin test can be done as part of urinalysis or as a separate dipstick test.
A positive dipstick test is usually followed by an albumin-to-creatinine ratio test,
which provides quantitative measurement of urine albumin
by comparing it to the amount of creatinine in the urine sample.
Another parameter of kidney function is creatinine clearance, the volume of blood plasma cleared of
creatinine per minute. Creatinine clearance test compares the creatinine in total urine
collected over 24 hours, to serum creatinine. It can also be estimated based on serum creatinine
adjusted for age, gender and weight. Imaging tests such as ultrasound and CT
scan are used to detect obstructions, kidney stones, tumors and abnormal kidney size.