Proteinuria is a general term that describes the presence of any type of protein in the urine (e.g., albumin, globulins, mucoproteins, and Bence-Jones proteins); however, albumin is the predominate protein in urine in healthy dogs and cats as well as dogs and cats with renal disease.  Proteinuria can arise from several different physiologic and pathologic causes, but persistent proteinuria associated with normal urine sediment is consistent with kidney disease.  
The urine dipstick colorimetric test is the usual first-line screening test for the detection of proteinuria, but false-positive reactions are common.  When proteinuria of renal origin is suspected, the next diagnostic steps are quantitation and longitudinal monitoring via the urine protein/creatinine ratio.  The recent availability of a species-specific albumin enzyme-linked immunosorbent assay technology that enables detection of low concentrations of canine and feline albuminuria has both increased diagnostic capability and stimulated discussion about what level of proteinuria/albuminuria is normal.  

Beyond being an important diagnostic marker, proteinuria is associated with kidney disease progression in both dogs and cats: the greater the magnitude of the proteinuria, the greater the risk of renal disease progression and mortality.  Treatments that have attenuated proteinuria in dogs and cats have also been associated with slowed kidney disease progression and/or improved survival.  For these reasons, screening for renal proteinuria and longitudinal assessment of renal proteinuria has recently received renewed interest.