Interference with serum indices measurement, but not chemical analysis, on the Roche Modular by Patent Blue V

Background: Patent Blue V is an inert dye increasingly being used during cancer surgery to identify the sentinel lymph node. We recently discovered three cases with falsely elevated lipaemic indices on the Roche Modular, following intramammary injection of Patent Blue. This, and other potential interferences by Patent Blue, is examined in this study. Methods: Serum samples which visually appeared normal, haemolysed or icteric were spiked with Patent Blue to a concentration of 7.3 mg/L and the effect on the lipaemic, haemolysis and icteric indices and routine chemical analyses (sodium, potassium, urea, creatinine, bilirubin, alanine aminotransferase, gamma-glutamyltransferase, aspartate aminotransferase, magnesium, creatine kinase, lactate dehydrogenase, amylase, albumin, calcium and phosphate) undertaken. Dose-response curves in the range of 4.9 mg/L to 22.7 mg/L of Patent Blue were established for normal, icteric and haemolysed pooled serum samples. Results: Significant positive interference by 7.3 mg/L of Patent Blue was observed in the lipaemic index, with an increase of 149 +/- 6.3 (mean +/- standard deviation [SD]; P < 0.001). Significant negative interference was observed in both the haemolysis and icteric indices, with decreases of 71 +/- 9.7 (P < 0.001) and 46 +/- 8.9 (P < 0.001), respectively. Patent Blue had a linear dose-response effect on the serum indices. No significant differences were observed in the chemical analyses assessed. Conclusions: The effects observed on the Roche Modular serum index measurements with Patent Blue raises the novel concept of interference in methods routinely used to detect interference. Despite Patent Blue having no direct effect on chemical analysis, failure to reliably detect potential interference, particularly haemolysis, may lead to misleading results being issued.