title: Clinical utility of testing AQP4-IgG in CSF: Guidance for physicians.
abstract: 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.
  
  
  METHODS
  This study provides Class IV evidence that for patients with NMO or NMOSD, CSF is less sensitive than serum for detection of AQP4-IgG.
authors.affiliation: Departments of Laboratory Medicine and Pathology (M.M., J.P.F., A.M., V.A.L., S.J.P.), Neurology (A.M., V.A.L., S.J.P.), and Immunology (V.A.L.), Mayo Clinic, Rochester, MN.
authors.forename: Masoud
authors.initials: M
authors.lastname: Majed
authors.affiliation: Departments of Laboratory Medicine and Pathology (M.M., J.P.F., A.M., V.A.L., S.J.P.), Neurology (A.M., V.A.L., S.J.P.), and Immunology (V.A.L.), Mayo Clinic, Rochester, MN.
authors.forename: James P
authors.initials: JP
authors.lastname: Fryer
authors.affiliation: Departments of Laboratory Medicine and Pathology (M.M., J.P.F., A.M., V.A.L., S.J.P.), Neurology (A.M., V.A.L., S.J.P.), and Immunology (V.A.L.), Mayo Clinic, Rochester, MN.
authors.forename: Andrew
authors.initials: A
authors.lastname: McKeon
authors.affiliation: Departments of Laboratory Medicine and Pathology (M.M., J.P.F., A.M., V.A.L., S.J.P.), Neurology (A.M., V.A.L., S.J.P.), and Immunology (V.A.L.), Mayo Clinic, Rochester, MN.
authors.forename: Vanda A
authors.initials: VA
authors.lastname: Lennon
authors.affiliation: Departments of Laboratory Medicine and Pathology (M.M., J.P.F., A.M., V.A.L., S.J.P.), Neurology (A.M., V.A.L., S.J.P.), and Immunology (V.A.L.), Mayo Clinic, Rochester, MN.
authors.forename: Sean J
authors.initials: SJ
authors.lastname: Pittock