The influence of oseltamivir carboxylate and oseltamivir on hemagglutinin inhibition and microneutralization test

The administration of oseltamivir in humans is suggested to affect the results of hemagglutinin–inhibition test. To investigate this phenomenon, the concentrations of oseltamivir and oseltamivir carboxylate (OC) in sera obtained from oseltamivir-administered individuals were quantified by the liquid chromatography–tandem mass spectrometry (LC–MS/MS) system. The analysis revealed that the concentrations of OC in sera obtained at 4 and 7 h after administration were greater than those at 24 h after administration. Flow cytometry analyses revealed that OC or oseltamivir added in the sera affects the expression level of sialic acid α2,3-Gal linkages on horse erythrocytes; however, no effect was observed on the expression level of these linkages on chicken erythrocytes. Moreover, the addition of oseltamivir or OC may yield pseudopositive results in hemagglutinin–inhibition assays. These results suggest that the pseudopositive results obtained in hemagglutinin–inhibition assays occurred by the presence of OC, and that it is very important to take care of the patients in the prescription of oseltamivir when anti-influenza investigations are performed.