To determine frequency of testing for neutralising antibodies (NAbs) to interferon-beta (IFNB) in patients with multiple sclerosis (MS) and adherence to international guidelines.

IFNB was the first approved disease modifying therapy (DMT) in MS and is still used in a significant proportion of the DMT population. The presence of NAbs affects the efficacy of treatment.  Guidelines recommend testing for NAbs after 12 and 24 months of treatment, with sustained high-titre NAb leading to reconsideration of treatment options.

In August 2019, the Northern Ireland (NI) DMT database was searched for patients receiving IFNB.  Patients currently or previously treated with IFNB were identified.  The NI Electronic Care Record (NIECR) and the regional laboratory database were reviewed for all to check if testing had been sent and the result.  NIECR was further reviewed for those with a positive result to establish outcomes. Reasons for cessation were studied.

488 patients were on an IFNB  in NI, 21.6% of the overall DMT population (210 on IFNB 1a intramuscularly, 71 on pegylated IFNB 1a, 175 on IFNB 1a subcutaneously, 32 on IFNB 1b subcutaneously).  Of these, 98 (20.1%) patients had ever had NAbs checked, with 11 (11.2%) being positive.  A further 288 patients had ceased treatment in the past 13 years, with 273 having available records. Of these, 32 (11.7%) had ever had NAb testing, with 7/32 (21.9%) testing positive. 62/273 patients had stopped treatment due to relapse or disease progression with only 9 having had NAbs checked.

The immunogenicity of IFNB is significant, (25% depending on compound). NAbs are readily measurable and a sustained high level can render treatment ineffective. These data suggest that guidelines on NAb testing are poorly observed in this population.  Improving testing should be prioritised, identifying patients at risk of treatment failure earlier and facilitating more effective DMT decision making.