Table 3 Recommendations on methodology, test parameters, specimen sampling and data interpretation

Assay types

Cell-based assays (IFT/FACS): Recommended (current gold standard); must employ full-length human MOG as target antigen; use of Fc-specific (or IgG1-specific [63]) secondary antibodies highly recommended to avoid cross-reactivity with (specifically or non-specifically co-binding) IgM and IgA antibodies [11, 63]

Immunohistochemistry: Currently not recommended (less sensitive than cell-based assays, limited data available on specificity [11, 64], sensitivity depends on tissue donor species [64]); if used, Fc-specific secondary antibodies adsorbed against tissue donor IgG required in order to avoid cross-reactivity with IgM and IgA or with tissue-bound donor IgG

Peptide-based ELISA, Western blot: Insufficiently specific, obsolete

Biomaterial

Serum: Recommended (specimen of choice); shipment at 4 °C or on dry ice advisable if samples do no arrive within 1–2 days

Cerebrospinal fluid: Not usually required, since MOG-IgG is produced mostly extrathecally, resulting in lower CSF than serum titers [2]; potentially helpful in rare, selected cases (e.g., strong background due to co-existing high-titer non-MOG serum antibodies); shipment at 4 °C or on dry ice advisable

Immunoglobulin classes

Testing for MOG-IgG: Recommended

Testing for MOG-IgM and/or MOG-IgA: Currently not recommended; additional MOG-IgM and MOG-IgA antibodies have been described in some MOG-IgG-positive patients [1, 2]; the clinical relevance of isolated MOG-IgM or -IgA results is unknown; testing for antibodies of the IgM class requires removal of total IgG from the sample to avoid both false-negative (due to high-affinity IgG displacing IgM) and false-positive (due to IgM anti-IgG_Fc rheumatoid factors) results [65]

Data reporting

Immunoglobulin class detected, assay type, antigenic substrate and biomaterial used, titer/concentration/units, assay-specific cut-offs and performing laboratory should all be documented (e.g., “Serum MOG-IgG 1:1280 [cut-off ≥ 1:160^a; assay: live CBA, Innsbruck lab; antigen: full-length human MOG]”)

Data interpretation

As with all laboratory tests, positive test results should always be interpreted in the context of the patient’s overall presentation; if “red flags” as defined in Table 4 are present, re-testing of the positive serum sample (or, if not anymore available, at least testing of a follow-up serum sample) is recommended; to reduce the potential risk of reproducing false-positive results due to issues inherent to the very method employed, use of a second (and, in the case of discrepant results, third) methodologically different cell-based assay is advisable; if in doubt, seek expert advice from a specialized center

Timing issues

MOG-IgG serum concentrations depend on disease activity (with higher median concentrations during acute attacks than during remission) and treatment status (with lower concentrations while on immunosuppression) and may transiently vanish after plasma exchange [3]; if MOG-IgG is negative but MOG-EM still suspected, re-testing during acute attacks, during treatment-free intervals, or 1–3 months after plasma exchange (or IVIG^b) is recommended – N.B.: Some cases of monophasic MOG-positive EM/ADEM in adult patients have been described in which MOG-IgG disappeared permanently following clinical recovery [2–4, 33,34,35]