Abstract

Background and aim Lymphocytic myocarditis, characterized by lymphocyte-predominant myocardial inflammation with associated myocyte injury, is a term that has decades-old histopathologic criteria when encountered on endomyocardial biopsy. However, the interpretation of non-biopsy specimens such as surgical resections and autopsy samples has lacked standardized histopathologic criteria, despite their growing clinical and forensic relevance. The aim was to develop and establish criteria for the diagnosis and classification of lymphocytic myocarditis in non-biopsy ventricular myocardial specimens. Methods and results An international panel of cardiovascular pathologists representing the Society for Cardiovascular Pathology (SCVP) and the Association for European Cardiovascular Pathology (AECVP) developed a new classification system, which was completed at a final meeting in the Seaport area of Boston. These “Seaport” criteria for non-biopsy specimens formally define lymphocytic myocarditis as myocardial inflammation predominantly composed of lymphocytes, accompanied by myocyte injury not attributable to other causes. Recommendations address specimen type, technical handling, diagnostic thresholds, and qualifiers of chronicity. Diagnostic categories include active myocarditis and lymphocytic infiltrate of uncertain significance (LIUS). The document also outlines the interpretive challenges in attributing causality in autopsy settings, provides guidance on the use of ancillary techniques, and highlights the limitations of current histopathologic approaches. Conclusion These consensus-based criteria offer a standardized framework for diagnosing lymphocytic myocarditis in non-biopsy specimens. Adoption of these guidelines is expected to improve diagnostic consistency, enhance research comparability, and inform clinical and forensic evaluations. Future efforts should aim to refine definitions of myocyte injury, validate ancillary techniques, and elucidate the clinical significance of inflammation in the absence of injury.