Background: Incorporation of routine clinical testing for neutralizing antibodies (NAbs) to interferon (IFN)-β has remained problematic. With increasing treatment choice for patients, routine NAb testing should be incorporated to aid therapeutic decisions.

Objective: We sought to improve interpretation of NAb results by combining the luciferase NAb assay (luciferase gene expression assay under control of interferon-stimulated response element) and in-vivo biomarker (myxovirus A protein, MxA) induction in patients with MS.

Methods: Blood samples (serum and PAXGene(®) for RNA) were obtained pre-injection and 12 hours post-injection of IFN-β from 144 subjects. Sera were tested for NAbs using the luciferase assay. MxA expression was quantified by real-time polymerase chain reaction (PCR).

Results: 26% of samples were NAb positive (titre > 20 NU). There was no difference in NAb titres in the pre- or post-dose sera (p = 0.643). MxA expression was inhibited in a dose-dependent fashion in NAb positive samples. Mean MxA level post-IFN-β: NAb negative 2330 (95% CI 1940-2719), NAb 20-99 NU 1533 (95% CI 741-2324), NAb 100-600 NU 832 (186-1478) and NAb > 600 NU 101 (95% CI 0-224). NAb titre and MxA level correlated strongly: MxA pre- (Spearman r = -0.72, p < 0.0001), MxA post- (Spearman r = -0.79, p < 0.0001) and MxA induction (Spearman r = -0.67, p = 0.0004).

Conclusions: A single, 12-hour post-injection sample should be used to test for NAbs using the luciferase assay and IFN-β bioactivity (MxA) in the clinical setting.