Retrospective Spanish cohorts find neurological GAD65 disease in 21.3% of high-titer patients but no low-titer cases

In a retrospective observational study at Hospital Universitari de Bellvitge in Spain, 32 of 150 patients with high serum glutamic acid decarboxylase 65 antibody titers, defined as at least 2,000 IU/mL, had confirmed neurological GAD65 antibody-associated disease. By comparison, none of 194 patients with lower titers had the condition during 20 to 25 years of follow-up, according to the study.

Researchers compared two cohorts drawn from the hospital’s diabetes and immunology databases. The low-titer group comprised consecutive patients whose diabetes began between 2000 and 2005 and who had positive GAD65 antibody results below 2,000 IU/mL. The high-titer group included patients whose laboratory results reached or exceeded 2,000 IU/mL between 2013 and 2024. The analysts used the Mann–Whitney U test, chi-square or Fisher’s exact tests, and multivariable logistic regression to compare clinical and immunological characteristics.

Among the 150 high-titer patients, 32 (21.3%) had a neurological syndrome classified as GAD65 antibody-associated disease. Epilepsy was the most frequent presentation, accounting for 13 of the 32 cases (40.6%). Other classifications included stiff-person syndrome in 10 patients, overlapping neurological syndromes in six, cerebellar ataxia in two and ophthalmoparesis in one. One patient with a syndrome resembling progressive encephalomyelitis with rigidity and myoclonus also had antibodies against the glycine receptor, CV2 and Zic4, as well as a malignant thymoma; the investigators considered the glycine-receptor antibodies the most likely explanation and did not classify the case as GAD65-associated disease.

Diabetes was present in 128 high-titer patients (85.3%), including 126 with diabetes associated with latent autoimmune diabetes in adults and two with immune checkpoint inhibitor-induced diabetes. Seventeen of the 128 patients with diabetes (13.3%) had neurological GAD65 disease. Conversely, 112 of the 118 high-titer patients without that neurological involvement had diabetes (94.1%). Diabetes occurred in 9 of 13 patients with epilepsy (69.2%), four of 10 with stiff-person syndrome (40.0%) and two of six with overlap syndromes (33.3%), but the differences between syndromes were not statistically significant.

Neurological disease began at a younger mean age among high-titer patients with diabetes: 43.8 ± 13.5 years, compared with 55.4 ± 14.1 years among those without diabetes (p = 0.03). Patients with neurological involvement also had higher GAD65 antibody titers than those without it (Mann–Whitney U test, p = 0.013). Both groups had a median titer of 2,000 IU/mL, but the distribution among affected patients extended to markedly higher values, with a third quartile of 41,357 IU/mL rather than 2,000 IU/mL. The observed ranges were 2,000–7,320,000 IU/mL in patients with neurological disease and 2,000–1,580,000 IU/mL in those without it.

Clinical findings did not differ significantly by sex: neurological disease affected 13 of 60 men (21.7%) and 19 of 90 women (21.1%). In the multivariable analysis, male sex was associated with lower odds of being in the high-titer group rather than the low-titer group (odds ratio 0.363, 95% confidence interval 0.218–0.606; p < 0.001), while age was not independently associated with titer group. Diabetes, neurological disease and the number of additional autoimmune diseases could not be estimated in that model because of complete separation, with zero or near-zero frequencies in the low-titer cohort.

Cerebrospinal fluid samples were available for only 11 high-titer patients, including 10 with neurological disease. GAD65 antibodies were detected in cerebrospinal fluid in eight patients by both assays used, and two additional cases were positive by one method and confirmed at the reference laboratory. Oligoclonal bands were present in six of seven patients for whom those data were available, and these six also showed intrathecal antibody synthesis. Because cerebrospinal fluid testing was limited and incomplete, the findings do not establish its diagnostic performance across the entire high-titer cohort.

The investigators noted that earlier series had reported neurological disease in 82% of high-titer patients and in 94% of those with titers above 10,000 IU/mL. They suggested that the lower 21.3% observed here may reflect a broader, less selected population drawn from a laboratory database rather than patients already referred to neurological or endocrine clinics. They also found patients with epilepsy and titers below 10,000 IU/mL, indicating that a higher cutoff may improve specificity at the expense of sensitivity.

Based on these observational data, the authors recommend serial serum GAD65 antibody testing for patients with diabetes and high titers, together with prompt neurological referral when symptoms arise. They emphasized that the recommendations rest on a retrospective dataset with a limited number of neurological events. Other limitations included incomplete variables, nonuniform laboratory methods, cerebrospinal fluid testing only in symptomatic patients, and the fact that not every high-titer patient was evaluated by a neurologist trained in epilepsy or movement disorders. Prospective studies with standardized assessments were therefore needed to confirm the observed associations and identify more reliable predictors of neurological involvement.

Subject of Research: Clinical and neurological characteristics associated with high and low serum GAD65 antibody titers in retrospective patient cohorts

Article Title: Clinical characteristics associated with GAD65 antibodies: a comparative study between high and low GAD65-ab cohorts

Article References: Lombardo-Del Toro, P., Jaraba, S., Hernández-Pérez, G., Nedkova, V., Casasnovas, C., Jauma-Classen, S., Gonzalez-Mera, L., Gómez-Llopico, R., García-Serrano, L., Sala-Padró, J., Morandeira, F., & Falip, M. (2026). Clinical characteristics associated with GAD65 antibodies: a comparative study between high and low GAD65-ab cohorts. Frontiers in Neurology, 17, Article 1976777. https://doi.org/10.3389/fneur.2026.1976777

Image Credits: AI Generated

DOI: 10.3389/fneur.2026.1976777

Keywords: GAD65 antibodies, neurological autoimmunity, type 1 diabetes, stiff-person syndrome, epilepsy, cerebellar ataxia, autoimmune diabetes, retrospective cohort, antibody titers, Clinical, characteristics, associated