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

Area of Science:

• Clinical Chemistry
• Urinalysis
• Diagnostic Accuracy

Background:

• Tetrabromphenol blue dye is used in urinalysis dipsticks for proteinuria detection.
• Manufacturers claim pH and specific gravity can cause false positive or negative results, but evidence is equivocal.

Purpose of the Study:

• To investigate the impact of urine pH and specific gravity on protein detection using tetrabromphenol blue dipsticks.
• To determine if common dipstick interferences are clinically relevant.

Main Methods:

• Three tetrabromphenol blue dipsticks (iChem 10SG, iChem Velocity, Multistix 8SG) were tested.
• Urine pools were adjusted for pH using NaOH, Na2CO3, or NaHCO3, and for specific gravity using NaCl.
• Protein, pH, specific gravity, electrolytes, and total CO2 were measured.

Main Results:

• Protein detection was unaffected by NaOH-induced alkalinization up to pH 10.9.
• False positive protein results occurred with Na2CO3 at pH 9.9 (>97 mmol/L total CO2) and NaHCO3 at pH 7.6 (>137 mmol/L total CO2).
• False negative protein results occurred when ionic strength exceeded 1100 mmol/L due to NaCl.

Conclusions:

• Tetrabromphenol blue dipsticks demonstrate robust proteinuria detection within physiological urine pH, total CO2, and ionic strength.
• The tested dipsticks are reliable for clinical use, as extreme conditions causing false results are rare in fresh patient samples.