The most common primary diagnoses, except for neuromyelitis spectrum disorder, all share the lack a specific biomarker, Dr. Solomon noted. Most of them require clinical skills and critical thinking to make the diagnosis, he said.
Most (70%) patients had received immunomodulatory treatment for MS, including 36% with more than one disease-modifying therapy. The treatments included natalizumab (Tysabri) in 13%, dimethyl fumarate (Tecfidera) in 6%, and fingolimod (Gilenya) in 5% – all of which have known risk for progressive multifocal leukoencephalopathy – as well as mitoxantrone in two patients and cyclophosphamide in one. Overall, 29% of those who received immunomodulatory therapy used it for 3-9 years, and another 29% used it for 10 or more years.
Dr. Solomon acknowledged selection and referral bias as limitations to the study, as well as possibly incorrect alternate diagnoses. The disease duration and hindshight bias also may have allowed a correct diagnosis to come to light. The study could not provide information on the frequency of misdiagnosis but “provides rationale for such data in the future,” he said.