Veterinary Clinical Biochemistry and Laboratory Diagnosis-II

Kidney Function tests I

KIDNET FUNCTION TESTS - I

Glomerular filtration rate (GFR)

• This is the most frequent test of renal function.
• GFR varies as a function of normal physiology as well as disease.
• Its measurement is based on determining the volume of plasma from which a substance is removed by glomerular filtration during its passage through the kidney.
• In other words, the clearanceof that substance, Clearance = (U x V) / P Where U = urinary concentration of X; V = rate of urine formation (ml/min); P = plasma concentration of X.

Creatinine Clearance

• The plasma creatinine concentration (alone) is only a very rough guide to renal function.
• It is often used as a rough measurement of GFR, with a timed urine collection (often 24 hrs).
• And a blood sample taken to measure plasma creatinine during that time period.
• It is limited by problems of accurate urine collection and tends to overestimate the GFR.
• Levels are affected by age, gender, ethnic group, muscle bulk, ingestion of cooked meat, malnutrition and after use of some drugs.

Creatinine clearance test

• This test evaluates how efficiently the kidneys clear a substance called creatinine from the blood.
• Creatinine clearance test is very specific measurement of kidney function and often used as a rough measurement of GFR. 
• The test is performed on a timed urine specimen—a cumulative sample collected over a two to 24-hour period.
•  Determination of the blood creatinine level is also required to calculate the urine clearance.

Urea clearance test

• Urea is a waste product that is created by protein metabolism and excreted in the urine.
• The urea clearance test requires a blood sample to measure the amount of urea in the bloodstream and two urine specimens, collected one hour apart, to determine the amount of urea that is filtered, or cleared, by the kidneys into the urine.

Urine osmolality test

• Urine osmolality is a measurement of the number of dissolved particles in urine.
• It is a more precise measurement than specific gravity for evaluating the ability of the kidneys to concentrate or dilute the urine.
• Kidneys that are functioning normally will excrete more water into the urine as fluid intake is increased, diluting the urine.
• If fluid intake is decreased, the kidneys excrete less water and the urine becomes more concentrated.
• The test may be done on a urine sample collected first thing in the morning, on multiple timed samples, or on a cumulative sample collected over a 24-hour period.
• The patient will typically be prescribed a high-protein diet for several days before the test and be asked to drink no fluids the night before the test.

Urine protein test

• Healthy kidneys filter all proteins from the bloodstream and then reabsorb them, allowing no protein, or only slight amounts of protein, into the urine.
• The persistent presence of significant amounts of protein in the urine, then, is an important indicator of kidney disease.
• A positive screening test for protein (included in a routine urinalysis ) on a random urine sample is usually followed up with a test on a 24-hour urine sample that more precisely measures the quantity of protein.