Signs/symptoms of underlying disease (e.g. stigmata of cirrhosis, wasting)
Third spacing of fluids due to decreased oncotic pressure (e.g. peripheral edema)
Hypoalbuminemia
Liver disease e.g. cirrhosis 
Proteinuria
Excess renal excretion e.g. nephrotic syndrome, glomerulonephritis
Abnormally colored urine
Excess bowel loss e.g. protein-losing enteropathy in Crohn's
Coagulopathy 
Burns 