Validation of a virus neutralization potency test in BHK-21 cells for rabies immunoglobulins in a two-center study

A rabies virus neutralization potency test (VNPT), adapted to microplates from the rapid fluorescent focus inhibition test (RFFIT) for rabies therapeutic immunoglobulin potency evaluation, was standardized and validated in a two-center study in Brazil. The two institutes involved in the study were: Instituto Nacional de Controle de Qualidade em Saúde (Fundação Oswaldo Cruz) and Instituto Butantan.Two equine rabies immunoglobulin samples, all diluted to 1 IU/ml, were tested against the WHO 2nd Rabies Human Ig International Standard. Four dilutions of the samples and standards were tested with the VNPT.The potency of the samples was calculated in IU/ml using the probit method; linearity, accuracy, repeatability (intra-assay variation), intermediate precision (inter-assay variation) and reproducibility (inter-laboratory variation) were assessed to evaluate the reliability of the VNPT. Laboratories were arbitrarily coded as Laboratory A and Laboratory B.The following results were obtained with the International Standard: (a) linearity, the overall coefficient of correlation of the dose–response curve was −0.97; (b) accuracy, % error of −0.70 (IU/ml); (c) repeatability, 17.06% (Laboratory A) and 11.61% (Laboratory B); (d) intermediate precision, 16.99% (Laboratory A) and 22.05% (Laboratory B); (e) reproducibility, 14.5%. The final conclusion was that VNPT presents satisfactory linearity, accuracy, repeatability, intermediate precision and reproducibility and is a reliable and suitable method by which to evaluate rabies immunoglobulin potency.