Do broad neural autoantibody panels need cautious interpretation in low-probability settings?

Broad neural autoantibody panels are most vulnerable to misleading interpretation when used outside a compatible autoimmune encephalitis or paraneoplastic neurologic phenotype. In this 71-year-old woman, as in any patient, a positive laboratory result should support—not define—the diagnosis.

Clinical evidence

• Low pretest probability substantially reduces the positive predictive value of broad commercial testing. In a cohort of 20,940 patients, only 31% of serum-only positive results occurred in patients with probable/definite autoimmune encephalitis or paraneoplastic neurologic syndrome, versus 91% of CSF-only positives.
• In another cohort, 39 of 63 positive results (62%) were judged nonspecific; alternative diagnoses were identified in 35 patients.
• Predictive value is antibody- and result-strength-dependent. For serum GAD65, the overall PPV for neurologic autoimmunity was 44%, but reached 97.7% at high concentrations.

Specimen and assay interpretation

• Paired serum and CSF testing is often more informative than serum alone. CSF is particularly important for suspected NMDAR or GFAP autoimmunity, while serum may be more sensitive for LGI1 and CASPR2.
• Isolated serum positivity, especially if low-level or syndrome-discordant, should not be considered diagnostic.
• Assay choice matters: cell-based assays are suited to conformational surface antigens; tissue-based assays can corroborate neuronal reactivity; immunoblot is used for intracellular targets. A commercial line-blot result without tissue-assay confirmation is particularly concerning: only 8% of tissue-assay-negative, line-blot-positive high-risk paraneoplastic antibody findings were true positives.

Clinical implications

• Antibody target/class, specimen, assay method, signal strength or titer, and fit with the neurologic syndrome must converge before attributing symptoms to autoimmunity.
• Unexpected, discordant, serum-only, or low-level positives warrant independent antigen-appropriate confirmation and reassessment for alternative diagnoses.

Bottom line

• The claim is well supported: broad commercial panels can produce false-positive or clinically irrelevant findings in low-probability settings; phenotype-led testing and cautious, method-aware interpretation are essential before diagnosis or immunotherapy.