Azotemia

The term azotemia is used to refer to increased serum urea and/or creatinine and it may be classified as prerenal, renal or postrenal. Prerenal azotemia occurs when there is reduced renal blood flow and therefore decreased GFR leading to decreased urea and/or creatinine excretion. Concentrated urine is expected and defined as a USG of >1.030 in dogs, >1.035 in cats and >1.025 in large animals. Although decreased blood volume (hypovolemia) due to dehydration is the most common cause of prerenal azotemia, other causes of hypovolemia include shock, blood loss, and hypoadrenocorticism. Decreased cardiac output (e.g. cardiac insufficiency) may also cause prerenal azotemia.

Renal azotemia occurs when renal pathology causes the serious decline in GFR. In this case, the urine will not be concentrated and the specific gravity may have reached isosthenuria (1.008 – 1.012). The decreased GFR causes reduced renal excretion of urea and/or creatinine hence, their serum concentrations rise.

Postrenal azotemia occurs when decreased excretion of urea and/or creatinine is caused by pathology distal to the nephron. Causes of postrenal azotemia include: urinary tract obstruction as may be seen with urolithiasis, urethral plugs in cats, neoplasia, or prostatic disease; and leakage of urine from the urinary tract into the abdomen (uroabdomen).