Background and purpose: Neutralizing antibody (NAb) titres may vary over time and thereby some patients can either lose or regain therapeutic effect of interferon beta (IFNbeta). We assessed NAb titres in a large sample of multiple sclerosis (MS) patients to identify the pattern of fluctuation during 1-3 years.
Methods: Data from IFNbeta-treated MS patients who had been tested for NAbs twice (n = 822) were analysed. NAb titres were compared between the first and the second samples across the critical NAb level of 150 TRU/ml, which we have previously reported to correlate with loss of bioactivity.
Results: Of patients with NAb titres high enough to indicate loss of bioactivity (>150 TRU/ml) in the first analysis 15% showed titres low enough to indicate a regained bioactivity. Conversely, 6% of those without detectable NAbs or NAb titres below the critical level of 150 TRU/ml had shifted to titres above this limit, indicating potential loss of bioactivity. Fluctuation did not differ between IFNbeta preparation used, treatment duration or sampling interval.
Conclusion: A first NAb test is prognostic for the NAb status during the coming 1-3 years. Choice of IFNbeta preparation had no influence on the chance of reverting to lower levels once NAb titres are high.