Objective: A combined thyroid transcription factor 1 (TTF-1) and Napsin A double stain has been shown to be useful in the diagnosis of adenocarcinoma (ADC). This study compares differences in double staining patterns among vendor antibodies (Leica, Dako, and Biocare). Study Design: The cohorts included 35 FNA cell blocks of lung ADC and 24 cell blocks of lung squamous cell carcinoma (SqCCA). Double-staining immunohistochemistry was performed with TTF-1 as a brown nuclear stain and Napsin A as a red cytoplasmic stain, using three sets of double stains. Additionally, FISH expression was performed on SqCCAs with aberrant TTF-1 expression. Results: The sensitivity for the double stains ranged from 40 to 74%, while the specificity ranged from 88 to 96%. Two Leica TTF-1-positive SqCCAs also showed low-level amplification by FISH assay not seen in the TTF-1-negative control SqCCAs. Conclusion: The use of Dako TTF-1 antibody paired with Leica Napsin A antibody as a double stain yielded the best results for diagnosing ADC; additionally, the Leica Napsin A-only staining results had the highest positive predictive value at 97%. Both Dako and Biocare antibodies expressed less staining of SqCCAs than Leica staining.

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