Glomerular permeability is affected by molecular weight, size, shape and electrical charge of molecules. It is also influenced by renal hemodynamics. In general, the renal threshold is 68,000 daltons, therefore the glomerular filtrate contains electrolytes (in a similar concentration to plasma) and low molecular weight proteins. These filtered proteins are normally taken up and catabolized by renal tubular epithelial cells, therefore altered renal tubular function can result in these proteins being retained in the urine (tubular proteinuria).
Alterations in the glomerular barrier from altered renal blood flow (e.g. passive venous congestion in congestive heart failure) may result in mild proteinuria (functional proteinuria), whereas glomerular disease will result in a significant and severe proteinuria, consisting mostly of albumin (pathologic glomerular proteinuria). Increased filtration of low molecular weight proteins that are in high concentration in blood (hemoglobin, myoglobin, Bence-Jones proteins) will result in a significant proteinuria as the high concentration of these proteins in the filtrate overwhelm the renal tubular resorptive capacity (overload proteinuria). Urinary or reproductive tract inflammation and hemorrhage will also result in proteinuria because of the contribution of plasma proteins.

Pre-renal proteinuria (also called overload or pre-glomerular proteinuria)

This occurs when there are large concentrations of small proteins which have molecular weights (MW) below the normal glomerular threshold, e.g. colostral proteins, Bence-jones proteins (free light chain of immunoglobulins), hemoglobin monomers or dimers and myoglobin.. These freely filtered proteins, when in excess, are not able to be completely resorbed by the renal tubules, resulting in proteinuria. A colostral proteinuria occurs in neonatal animals less than 40 hours old. Free light chains occur in excess in animals with plasma cell neoplasia, including extramedullary plasmacytoma and multiple myeloma. Hemoglobinuria will occur in states of intravascular hemolysis. There should be concurrent hemoglobinemia and red urine supernatant (if there is large amounts of hemoglobin) with a concurrent anemia if there is a proteinuria due to hemoglobinuria. Myoglobin will leak into the urine in conditions of severe muscle injury (rhabdomyolysis). Increased values of muscle enzymes (CK, AST) will be seen in chemistry panels from affected animals, although the urine may not be discolored unless the myoglobinuria is severe. Note that both hemoglobin and myoglobin will cause a positive result for heme proteins on the dipstick. But, the protein reaction on the dipstick is less sensitive to hemoglobin and myoglobin than it is to albumin.

Renal proteinuria

This occurs when there are alterations in glomerular permeability (glomerular proteinuria) or inability of the tubules to resorb small MW proteins which are normally filtered into the tubules through the glomerulus. Renal proteinuria also occurs when disease processes within the kidney cause leakage of serum protein into the urine, e.g. hemorrhage, inflammation.

• Glomerular proteinuria: This can be functional or pathological. Functional proteinuria results in a mild proteinuria and is due to increased hydrostatic pressure or an altered glomerular filtration coefficient, e.g. stress, exercise, fever, excitement, congestive heart failure. Pathological glomerular proteinuria is due to renal disease (e.g. renal amyloidosis, glomerulonephritis) and can be selective (resulting in loss of small MW proteins such as albumin) or non-selective (with loss of larger MW proteins, such as globulins) through the glomerulus in excess of the tubules ability to resorb them (in this case, there are no tubular abnormalities, at least initially). With glomerular disease, the proteinuria usually starts off as selective, with only albumin being lost (due to its small size). As the disease worsens, the glomerular filter barrier is further disrupted and higher MW proteins leak into the urine, resulting in a non-selective proteinuria. The dipstick result is usually 2+ or greater and excessive for the USG in pathologic glomerular proteinuria. The urine protein:creatinine ratio (UP:UC) is usually > 2 and often > 5 in dogs. This can result in hypoalbuminemia if urinary albumin loss is severe or prolonged.
• Tubular proteinuria: This is due to decreased renal tubule function resulting in decreased absorption of filtered low MW proteins or increased excretion of proteins by damaged tubules. This results in a mild to moderate proteinuria, i.e. dipstick readings of 2+ or less and UP:UC usually < 2 in dogs, and consists of low MW proteins with only small amounts of albumin. This does not result in hypoalbuminemia. Causes of tubular malfunction are many and varied, e.g. Fanconi syndrome, renal ischemia, nephrotoxins (such as aminoglycosides). In cases of tubular proteinuria, there is often other evidence of tubular malfunction in the urine sediment, e.g. inappropriate USG, cylindriuria, glucosuria (without hyperglycemia).
• Interstitial proteinuria: This is due to hemorrhage or inflammation within the kidney.

Post-remal proteinuria
This is due to hemorrhage or inflammation in the urinary tract (ureter, bladder, urethra) or in the reproductive tract that causes protein (in the serum that accompanies inflammation or hemorrhage) to enter the urine once it has been formed and entered the renal pelvis.

• Inflammation: Inflammation and/or infection anywhere in the distal urinary or genital tracts, e.g. cystitis, prostatitis, will cause proteinuria from leakage of serum protein along with leukocytes or due to increased vascular permeability. The UP:UC is an unreliable indicator of urinary protein loss in the presence of inflammation or infection (i.e. an active urine sediment).
• Hemorrhage: In severe hemorrhage, the dipstick pad will be unreadable due to the urine color. Hemorrhage frequently results in proteinuria when serum protein accompanies erythrocytes. Hemorrhage can occur anywhere within the urogenital tract (including the kidney itself, although proteinuria associated with renal hemorrhage would be called renal interstitial proteinuria) but more frequently reflects lower urinary tract (bladder disorders) or reproductive tract disease. Note that traumatic microscopic hematuria associated with cytocentesis does not usually result in proteinuria.