Background:In a subgroup of patients with multiple sclerosis natalizumab therapy causes generation of anti-natalizumab antibodies that may be transient or persistent. It is recommended to discontinue natalizumab therapy in persistently antibody-positive patients. Objective:To use titres of anti-natalizumab antibodies to predict persistency of antibodies. Patients and methods:In 525 consecutive natalizumab treated patients tested for anti-natalizumab antibodies 43 (8.2%) were antibody-positive. Thirty of the antibody-positive patients, who were tested both at three and at six months after treatment start, had antibody titres in blood measured using an extended ELISA method. Results:Samples from persistently positive patients ( N=18) had higher titre values than samples from transiently positive patients ( N=12). A cut-off value for high titre values was generated, above which patients may discontinue natalizumab therapy after three months. The method had a sensitivity of 0.83, a specificity of 1.00 and a diagnostic accuracy of 0.90. Conclusion:An extended ELISA method for measuring anti-natalizumab antibody titres in multiple sclerosis patients on natalizumab therapy may be used for evaluation of antibody persistence. A test at three months may identify patients with high titres, who should discontinue natalizumab therapy, and patients with transient low-titre antibodies, who may continue natalizumab therapy despite development of antibodies.