Analytical interference of quinolone antibiotics and quinine derived drugs on urinary protein determined by reagent strips and the pyrogallol red-molybdate protein assay

Objectives: To investigate the analytical interference of drugs in urinary protein and to estimate the lowest interfering concentrations. Design and methods: Drug supplemented urine samples were compared to the control with two reagent strips and the total protein was determined using Pyrogallol Red-Molybdate (PRM). Results: False-positive interferences occurred with Multistix (R) 10 SG for hydroxychloroquine, levofloxacin and ofloxacin. No interference was observed with Combur (R) 10 Test M. Statistically significant false-positive interferences were observed in the PRM assay with all tested drugs, and lowest interfering concentrations were mostly above estimated therapeutic concentrations. The PRM assay "confirmed" the results of the Multistix dipstick, so a real proteinuria could be presumed from the double analytical interference. Conclusions: This is the first report of analytical interference by quinolone and quinine derivatives in the PRM assay. Special attention to patients using these drugs is needed to minimize errors in the interpretation of urinary protein results. (C) 2011 The Canadian Society of Clinical Chemists. Published by Elsevier Inc. All rights reserved.