An evaluation of the interference of hydroxycobalamin with chemistry and co-oximetry tests on nine commonly used instruments.

The administration of hydroxocobalamin (OHCob), alone or with sodium thiosulfate, is a standard therapy for cyanide poisoning. OHCob is a red chromophore, and its interference with co-oximetric and colorimetric laboratory measurements has been evaluated in a few conflicting reports. The interference of OHCob was investigated in samples spiked with 10 different concentrations of OHCob (0-1500 mg/L). The concentration of 73 different analytes was measured using nine different analysers (ABL 800 Flex, Advia 1800, Advia Centaur Xp, Architect ci8200, Immulite 2500, Konelab 30i, Modular Analytics SWA, Synchron LX 20 and Vitros 5.1). All instruments yielded some results that were affected by OHCob at concentrations equivalent to a single therapeutic dose. Of the 73 different analytes, 64% showed interference on at least one instrument. Of all 187 tests performed, 47% were biased with more than 10%. Interference was generally limited to photometric assays, whereas immunological and ion-selective electrode measurements were unaffected. OHCob present in the blood after treatment for cyanide poisoning interfered with many laboratory assays in an unpredictable way, making some results invalid. Some affected tests are important in the treatment of cyanide poisoning. The interference is not solely due to wavelength, but also to chemical interaction. Without delaying the administration of OHCob, blood should, preferably, be drawn in advance, or, at least, the laboratory should be informed about the OHCob treatment. If the laboratory receives OHCob-containing samples, methods and instruments should be selected to minimize bias, and the manufacturer of the OHCob should recommend relevant precautions to customers in the package insert.