Proficiency test of BRAF immunohistochemistry as a surrogate marker of p.V600E mutation: Assessment of staining and interpretation quality in Taiwan.

BRAF immunohistochemistry (IHC) serves as a surrogate for BRAF p.V600E but shows variable performance across tumor types and institutions. This study evaluated BRAF IHC staining quality and interpretation in general pathology laboratories through a nationwide proficiency test (PT) in Taiwan, focusing on the most commonly encountered thyroid neoplasm and colorectal cancer. This PT was organized by the Taiwan Society of Pathology using a tissue microarray, containing six tumor cores with confirmed BRAF mutation status and one positive control. Participating laboratories performed BRAF IHC staining and interpretation independently, with results centrally reviewed for concordance, accuracy, and staining quality. Twenty-six pathology laboratories participated. Two laboratories failed the initial control check. Among the remaining 24, 17 (70.83%) demonstrated optimal staining, while 5 (20.83%) showed over-staining and 2 (8.33%) under-staining. No significant associations were found between staining quality and antibody clones, platforms, dilution folds, or assay types. Interpretation was highly concordant (100% agreement and accuracy) for tissues with 3 + or negative staining. However, discrepancies arose in tissues with 2 + intensity (50% positive, 41.67% negative, 8.33% equivocal) and 1 + intensity (83.33% disagreement with test results). The overall accuracy was 79.2%, with sensitivity at 58.3% and specificity at 100%. Under-calling was frequent in cases with 1 + staining (33 cores across 23 laboratories) and 2 + staining (3 cores across 3 laboratories). Our study highlights the importance of optimizing staining quality and reinforcing education on interpretation criteria. To minimize false-negative results, we recommend molecular confirmation for all cases exhibiting diffusely weak staining.