Albuminuria and Proteinuria
Basic Information 
Physiology
• The glomerular capillary wall restricts the filtration of most plasma proteins on the basis of protein weight and protein charge. Small (<68,000 d) electrically neutral or positively charged proteins are more readily filtered. In most healthy animals, the proteins that pass the glomerular filter are resorbed in the proximal tubules. Albumin is negatively charged and too large (69,000 d) to filter through the normal glomerulus.
• A small amount of albumin and Tamm-Horsfall protein, a mucoprotein secreted by the tubular and collecting duct cells, makes up most of the protein found in the urine of healthy animals.
Causes of Abnormally High Levels
• Prerenal (overflow): Caused by fever, stress, hemoglobinuria, myoglobinuria, and paraproteinuria
• Glomerular proteinuria: Caused by increased glomerular permeability due to autoimmune destruction, glomerulonephritis, and amyloidosis
• Tubular proteinuria: Caused by decreased protein resorption due to congenital or acquired proximal tubular diseases. This is a rare cause of proteinuria in horses.
• Hemorrhagic or inflammatory proteinuria
• Inflammation in regions other than the urinary tract may cause trace proteinuria.
• Transient postexercise proteinuria is common at submaximal and maximal levels of exertion.
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