Determining susceptibility of routine clinical biochemistry assays to methylene blue interference

Methylene blue (MB) is a blue-coloured treatment for multiple indications, including methemoglobinemia. Its interference potential with spectrophotometric and/or colorimetric clinical assays has not been well studied.

We determined an MB absorbance spectrum and identified spectrophotometric assays in our core laboratory with interference potential based on their wavelengths. We identified twelve Roche cobas c502/c702 chemistry assays and co-oximetry on the Werfen GEM Premier 4000 blood gas analyzers as potentially susceptible. We also tested colorimetric urinalysis performed on a Roche cobas u601 urine analyzer. For Roche cobas c502/c702 assays or urinalysis, we performed an initial study with MB mostly at concentrations of 10 or 100 mg/L (or water as a control) in leftover patient specimens with relevant analyte concentrations. For co-oximetry, venous whole blood from healthy donors was spiked with MB. For assays with differences between MB-spiked samples and controls exceeding total allowable error, dose-response studies were performed to better characterize the MB concentrations causing interference.

MB >0.1, >0.01, and >0.08 mg/L, respectively, caused negative interference with the cobas c502 hemolysis index, positive interference with the cobas c502 lipemia index, and positive interference with the cobas c702 urine amphetamines assay. MB at ≥50-70 mg/L in venous whole blood yielded incalculable co-oximetry results on the GEM Premier 4000. These interferences were not felt to be clinically concerning. However, a urine MB concentration of ≥5 mg/L interfered with most urinalysis parameters on the cobas u601 analyzer. Automated comments based on colour detection were introduced to make clinicians aware of potential MB interference with urinalysis.

We identified the assays in our core laboratory susceptible to MB interference and addressed them where clinically indicated.