Clinical utility of testing AQP4-IgG in CSF: Guidance for physicians

Objective: To define, using assays of optimized sensitivity and specificity, the most informative specimen type for aquaporin-4 immunoglobulin G (AQP4-IgG) detection. Methods: Results were reviewed from longitudinal service testing for AQP4-IgG among specimens submitted to the Mayo Clinic Neuroimmunology Laboratory from 101, 065 individual patients. Paired samples of serum/CSF were tested from 616 patients, using M1-AQP4- transfected cell-based assays (both fixed AQP4-CBA Euroimmun kit [commercial CBA] and live in-house flow cytometry [FACS]). Sensitivities were compared for 58 time-matched paired specimens (drawn ≤30 days apart) from patients with neuromyelitis optica (NMO) or high-risk patients. Results: The frequency of CSF submission as sole initial specimen was 1 in 50 in 2007 and 1 in 5 in 2015. In no case among 616 paired specimens was CSF positive and serum negative. In 58 time-matched paired specimens, AQP4-IgG was detected by FACS or by commercial CBA more sensitively in serum than in CSF (respectively, p = 0.06 and p < 0.001). A serum titer .1:100 predicted CSF positivity (p < 0.001). The probability of CSF positivity was greater around attack time (p = 0.03). No control specimen from 128 neurologic patients was positive by either assay. Conclusions: FACS and commercial CBA detection of AQP4-IgG is less sensitive in CSF than in serum. The data suggest that most AQP4-IgG is produced in peripheral lymphoid tissues and that a critical serum/CSF gradient is required for IgG to penetrate the CNS in pathogenic quantity. Serum is the optimal and most cost-effective specimen for AQP4-IgG testing. Classification of evidence: This study provides Class IV evidence that for patients with NMO or NMOSD, CSF is less sensitive than serum for detection of AQP4-IgG.