The nuts and bolts of proteinuria (Proceedings)
Persistent proteinuria of renal origin is an important marker of chronic kidney disease (CKD) in dogs and cats. Unfortunately, due to the high incidence of false-positive results for proteinuria on the urine dipstick screening test and proteinuria associated with lower urinary tract inflammation, positive reactions for urine protein are quite common and therefore often disregarded. Ruling out false-positive proteinuria and identifying proteinuria of renal origin are necessary first steps when evaluating the results of tests for proteinuria. In the case of CKD, albumin is usually the primary component of renal proteinuria. In addition to being a diagnostic marker for CKD, the potential for renal proteinuria/albuminuria to be a mediator of CKD progression also exists. The recent development of species specific albumin ELISA technology that enables detection of low concentrations of canine and feline albuminuria has stimulated discussion about what level of proteinuria/albuminuria is normal and what levels may be associated with renal disease progression. For these reasons, detection and monitoring of renal proteinuria in dogs and cats has recently received renewed interest. Perhaps somewhat similar to our changing definition and treatment guidelines for systemic hypertension, the need to recognize, monitor, and potentially treat renal proteinuria, that not long ago may have been considered normal, is increasing.
Screening tests for proteinuria:
Proteinuria is routinely detected by semi-quantitative, screening methods, like the conventional dipstick colorimetric test (very common) and the sulfosalicylic acid (SSA) turbidimetric test (less common). The dipstick test is inexpensive and easy to use. This test primarily measures albumin, however both the sensitivity and specificity for albumin are relatively low with the dipstick methodology. False-negative results (decreased sensitivity) may occur in the setting of Bence Jones proteinuria, low concentrations of urine albumin, and/or dilute or acidic urine. The conventional dipstick test has a sensitivity level of > 30 mg/dl. False-positive results (decreased specificity) may be obtained if the urine is alkaline or highly concentrated or the dipstick is left in contact with the urine long enough to leach out the citrate buffer that is incorporated in the filter paper pad. False-positive results with the dipstick occur more frequently in cats compared with dogs but are common in both species.The SSA test is performed by mixing equal quantities of urine supernatant and 3-5% SSA in a glass test tube and grading the turbidity that results from precipitation of protein on a 0 to 4+ scale. In addition to albumin, the SSA test can detect globulins and Bence Jones proteins. False-positive results may occur if the urine contains radiographic contrast agents, penicillin, cephalosporins, sulfisoxazole, or the urine preservative thymol. The protein content may also be overestimated with the SSA test if uncentrifuged, turbid urine is analyzed. False-negative results are less common in comparison with the conventional dipstick test due to the increased sensitivity of the SSA test for protein (> 5 mg/dl). Because of the relatively poor specificity of the conventional dipstick analysis, many reference laboratories will confirm a positive dipstick test result for proteinuria with the SSA test. Grading of both the color change on the dipstick test and the turbidity on the SSA test is subjective and therefore results can vary between individuals and laboratories.
Proteinuria detected by these semi-quantitative, screening methods has historically been interpreted in light of the urine specific gravity and urine sediment. For example, a positive dipstick reading of trace or 1+ proteinuria in hypersthenuric urine has often been attributed to urine concentration rather than abnormal proteinuria. In addition, a positive dipstick reading for protein in the presence of microscopic hematuria or pyuria was often attributed to urinary tract hemorrhage or inflammation. In both examples, the interpretation may not be correct. Given the limits of the conventional dipstick test sensitivity, any positive result for protein regardless of urine concentration may be abnormal (except in the case of false-positive results). Likewise, hematuria and pyuria have an inconsistent effect on urine albumin concentrations; not all dogs with hematuria and pyuria have albuminuria.