The kidneys are responsible for maintaining fluid and mineral balance in the body. Water and essential molecules are reabsorbed from the tubules and the remaining waste products are excreted as urine. Certain molecules, such as proteins, are kept in the blood stream by the filtering units of the kidney, the glomerulus. Glomeruli are small tufts of capillary blood vessels that act as a sieve, allowing small substances to pass through while keeping larger substances (protein, blood cells) in the bloodstream.
When glomeruli are damaged they can become leaky. When red and white blood cells and proteins are loss through the kidney it is a condition called glomerulonephritis. The excessive loss of protein through the glomerulus is called proteinuria.
Glomerular proteinuria may be divided into selective and nonselective proteinuria. With selective glomerular proteinuria large amounts of albumin are lost through the glomerulus. It results from a number of disorders but primarily from immune-mediated diseases and amyloidosis. The causes of nonselective glomerular proteinuria are the same as with selective glomerular proteinuria but results in a more severe proteinuria with loss of globulins, fibrinogen, antithrombin III (which is responsible for preventing blood clots) as well as albumin.
Diagnostic Testing In-depth When a positive reaction for protein is detected on a routine urinalysis, the urine sediment should be evaluated
If pyuria (white blood cells in urine) is present then culture the urine to determine which type of bacteria is present
If blood is present in the urine look for causes of hemorrhage
If positive for blood but no intact red cells are seen consider hemolysis (red blood cell destruction). A positive blood test can also indicate hemoglobin or myoglobin in the urine
If casts of kidney tubules are seen, a disease of the kidney should be considered
Perform a urine protein/creatinine (U P/C) ratio. Measures the amount of protein and creatinine in a single sample. The U P/C of a single random urine sample from dogs with normal and abnormal glomerular function correlates with a 24 hour collection determination. A ratio greater than 1 is considered abnormal.
A 24 hour total urine protein quantification can also be performed by collecting all the animal's urine over a 24 hour period. In general, urine protein excretion values less than 30 mg/kg/day can be considered normal for cats and dogs.
Once proteinuria is confirmed obtain a complete blood count (CBC) and serum biochemistry profile
If the blood albumin level is decreased suspect glomerular disease
Blood pressure determination is important to identify systemic hypertension (high blood pressure)
Clotting profile and antithrombin III levels are performed to determine if the pet is at risk for bleeding problems or at an excessive risk for developing a thromboembolus (blood clot)
Kidney biopsy ultimately is needed to identify glomerulonephritis from amyloidosis
For more information, please read glomerulonephritis.