Urine Total Protein Measurement with the Vitros Dry Reagent Technology: Modification of Diluent to Resolve Positive Bias of Diluted Samples

The measurement of urinary total protein continues to have a place in the repertoire of clinical laboratory tests, particularly in the clinical assessment of patients with renal disease. Although the test volume is often modest, there exists the need to perform the assay on the major automated chemistry analyzers currently in use in most hospital laboratories. We have evaluated the performance of a new urine total protein procedure developed for dry reagent slide technology for use on the Vitros 950 analyzer (Johnson & Johnson). In this method, protein in the sample binds to a pyrocatechol violet–molybdate complex, resulting in an absorption shift. The method is calibrated with aqueous solutions of bovine serum albumin supplied by Johnson & Johnson. We compared the new method with that in current use on the Cobas Fara analyzer (Hoffmann–La Roche), which involves pyrogallol red dye binding of urine total protein (1) calibrated with human serum albumin (Biotrol). For this comparison we analyzed 53 patient samples (median 0.65 g/L, range 0.02–5.44 g/L, pyrogallol red results) and obtained the following Deming linear regression analysis: r = 0.807, slope = 3.01, intercept = −1.19 g/L, Sy‖x = 0.72 g/L (x = Cobas Fara pyrogallol red, y = Vitros method). These results showed a strong positive bias with the Vitros method with respect to the pyrogallol red method. The differences between the procedures appeared greatest in samples that required dilution, i.e., those >2.00 g/L, the linear range of both methods. Dilutions, when required, were carried out with deionized water in accordance with both manufacturers’ instructions. Deming linear regression analysis of the samples that did not require dilution (n = 36, median 0.23 g/L, range 0.02–1.51 g/L, pyrogallol red results) gave the following: r = 0.980, slope = 1.16, intercept = 0.06 g/L, Sy‖x = 0.06 g/L. Thus the discrepancies between the two methods were markedly increased when dilutions were involved.