Are AQP4 and paraneoplastic neural antibody results most meaningful when confirmed and matched to a characteristic neurologic syndrome?

In a 71-year-old woman—an age group in which malignancy-associated neurologic presentations may be clinically relevant—AQP4-IgG and paraneoplastic neural-antibody findings should be treated as evidence to integrate with the syndrome, imaging/other evaluations, and alternative diagnoses, not as stand-alone diagnoses.

Clinical and diagnostic evidence

• For NMOSD, 2015 criteria require AQP4-IgG positivity, at least one core clinical characteristic, and exclusion of alternatives. Relevant presentations include optic neuritis, acute myelitis, area-postrema syndrome, and specified brainstem, diencephalic, or cerebral syndromes.
• For paraneoplastic neurologic syndromes (PNS), the 2021 PNS-Care framework combines the neurologic phenotype, antibody cancer-risk category, and tumor evidence/follow-up. Definite PNS generally requires a compatible phenotype, a high- or intermediate-risk antibody, and cancer evidence.
• Thus, a characteristic neurologic syndrome is not merely supportive context: it is a central diagnostic requirement.

Assay quality and confirmation

• Serum cell-based assays are preferred for AQP4-IgG. Low-positive or clinically discordant results should be confirmed with a different assay; low-titer ELISA results have greater false-positive concern.
• For neural-antibody panels, unexpected, low-titer, serum-only, or phenotype-discordant positives warrant independent confirmation, often including tissue-based and antigen-specific methods. In one cohort, PNS was identified in 91% of line-blot-positive samples concordant with immunohistochemistry, versus 8% when results were discordant.
• Broad panel testing can mislead in low-pretest-probability settings: one commercial line-blot analysis reported a positive predictive value of only 14.2%, with more false than true positives.

Bottom line

• AQP4-IgG or paraneoplastic antibody positivity is diagnostically meaningful only when technically credible and clinically concordant. An isolated panel-positive result cannot establish NMOSD or PNS; it should prompt phenotype-based assessment, exclusion of alternatives, appropriate cancer evaluation for suspected PNS, and confirmatory testing when the result is questionable.