Urine protein detection by dipstick: No interference from alkalinity or specific gravity

Tetrabromphenol blue dye based methods are used to detect proteinuria using urinalysis dipsticks. Manufacturers have claimed that alkalinity leads to false positive proteinuria, and that high specific gravity leads to false negative protein results. However, published reports describing this phenomenon remain equivocal. This study aimed to determine whether pH and/or specific gravity affect protein detection in patient urine using three different tetrabromophenol blue dye-based dipsticks. Patient urine pools were divided into individual aliquots with varied pH or specific gravity, and measured for protein in triplicate using iChem 10SG, iChem Velocity, and Multistix 8SG dipsticks. The pH experiment involved progressive alkalinization of urine aliquots with either 1M NaOH, Na₂CO₃, or NaHCO₃; pH was recorded by electrode. The specific gravity experiment involved mixing aliquots with NaCl and spiking with human albumin. Urine electrolytes and total CO₂ were measured (Roche cobas 8000). Fresh patient urines (N = 35) were analyzed for physiological urine pH and total CO₂. Urine protein results were not affected by NaOH alkalinization up to pH 10.9. False positive protein occurred at pH 9.9 and >97 mmol/L total CO₂ (Na₂CO₃ alkalization; P < .05). Moreover, false positive protein occurred at pH 7.6 when total CO₂ exceeded 137 mmol/L (NaHCO₃ alkalization; P < .05). Fresh patient urines did not exceed pH 8.5 or 86 mmol/L total CO₂. NaCl elevated specific gravity and caused false negative protein detection when urine ionic strength was >1100 mmol/L (P < .05). Tetrabromphenol blue dipsticks provide robust detection of proteinuria when human urine is within physiological pH, total CO₂ and ionic strength.