Absence of Detectable Anti‐ <scp>HDV</scp> Antibodies Among <scp>HBsAg</scp> ‐Positive Blood Donors, <scp>HIV</scp> ‐Infected Individuals and Patients With Chronic Hepatitis B in Sao Paulo, Brazil: Evaluation Using a Rapid Lateral Flow Assay

ABSTRACT Hepatitis Delta Virus (HDV) infection in HBV‐infected individuals is associated with severe clinical outcomes, and screening is recommended in HBsAg‐positive patients. We used a rapid lateral flow assay (LFA) to assess anti‐HDV antibodies in Brazil, where HDV is highly endemic in the North, while data from the Southeast, particularly São Paulo, remain limited. We conducted a retrospective cross‐sectional study to assess anti‐HDV antibodies among 46 HBsAg‐positive blood donors at Fundação Pró‐Sangue/Hemocentro de São Paulo, 381 patients with chronic hepatitis B from the Central Laboratory Division of HC‐FMUSP, and 189 individuals from the Hepatology Clinic of HC‐FMUSP (101 HIV‐infected and 88 chronically HBV‐infected). Anti‐HDV antibodies were detected using a validated LFA. Positive controls ( n = 24) included samples previously confirmed by ELISA and/or PCR, predominantly infected with HDV genotypes 3 and 8. Among 443,400 blood donations, 48 (0.01%) were positive for at least two HBV markers; of these, 46 were tested for HDV, and all were anti‐HDV negative. All 381 patients undergoing HBV viral load monitoring were anti‐HDV negative, as were all 189 patients receiving hepatology care, including those with HIV coinfection or advanced liver disease. The LFA correctly identified all control samples positive for both anti‐HDV antibodies and HDV RNA. The absence of detectable anti‐HDV among diverse at‐risk populations in São Paulo is consistent with the low endemicity of HDV infection in southeastern Brazil. These findings, together with the assay's high specificity in HDV RNA‐positive controls, support its potential utility as a screening tool for HDV infection in low‐prevalence settings.