Ab0047 Replication of the 23-Valent Polysaccharide Pneumococcal Vaccine-Induced Skin Pathergy Test in an Independent Cohort of Patients With Behçet Disease

Background: The skin pathergy test (SPT) is an important tool in the diagnosis of Behçet Disease (BD), but its decreasing sensitivity over the years has limited its use [1,2]. In our previous study, we showed a dramatic improvement in the sensitivity of SPT with the additional stimulation by the 23-valent polysaccharide pneumococcal vaccine (PS-23) antigens, especially in BD patients with active disease manifestations (aBD) as high as 80.3% with preserved specificity of 100% [3]. Objectives: The current study aimed to replicate our previous findings and improve its methodology by increasing the number of PS-23-induced pricks. Methods: This study was performed between March 2022 and November 2023. Information related to demographics, clinical findings, disease activity, and treatment were collected. BD patients, patients with other inflammatory diseases, and recurrent aphthous stomatitis comprised the study group. The application of SPT was demonstrated in Figure 1. Standard SPT was done by pricking the forearm with a 3-5 mm deep oblique insertion at a 30-45° angle and using 20G hypodermic needles (20G-SPT). PS-23-induced SPT (PS23-SPT) was applied by dropping a total volume of 10 μl PS-23 (Pneumovax) additionally before pricking by 20G needle. In total four pricks were applied to each arm (2 20G-SPT, 2 PS23-SPT). Development of erythema and induration at 48h were evaluated by the same observer. Induration (≥2 mm) with erythema at 48h was accepted as positive. Patients with at least one active disease manifestation were grouped as aBD. Results: Stimulation of forearm skin by PS23-SPT showed 79.6% sensitivity and 100% specificity in the analysis of all patients with BD, and 91.3% sensitivity and 100% specificity in aBD patients, compared to the sensitivity of 8.6% in all and 10% in aBD using the 20G-SPT method. Both erythema and induration results with PS23-SPT showed a significant correlation between the first and second prick sites of the arms, and between the right and left arms (p<0.0001, and r=0.7-0.85 for all comparisons). In aBD group, 28.6% of the PS23-SPT negative and 6.3% of PS23-SPT positive patients were under at least one immunosuppressive. Fifty out of 73 PS-SPT-positive aBD patients (68.5%) developed pustule in at least one of the prick sites. Conclusion: The current study replicated the results of the previous findings of increased sensitivity of PS23-SPT especially in active patients and in those not using immunosuppressives, and doubling of the prick numbers increased further the sensitivity of PS23-SPT. The PS23-SPT is being considered a promising tool for the diagnosis of BD, and an international replication study in different ethnic groups is underway. REFERENCES: [1] Davatchi F, et al. Diagnostic value of pathergy test in Behcet's disease according to the change of incidence over the time. Clin Rheumatol. 2011;30(9):1151-5. [2] Dilsen N, et al. Comparative study of the skin pathergy test with blunt and sharp needles in Behcet's disease: confirmed specificity but decreased sensitivity with sharp needles. Ann Rheum Dis. 1993;52(11):823-5. [3] Deniz R, et al. Improved sensitivity of the skin pathergy test with polysaccharide pneumococcal vaccine antigens in the diagnosis of Behcet disease. Rheumatology (Oxford). 2023;62(5):1903-9. Acknowledgements: NIL. Disclosure of Interests: All authors confirm that they have no competing interest regarding this manuscript. No funding support was received for study. The study protocol was approved by the Institutional Research Ethics Committee (Project number 2022/826), and all patients gave a written informed consent.