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To evaluate the diagnostic value of a novel fully automated immunohistochemistry (IHC) assay for detection of anaplastic lymphoma kinase (ALK) fusion in a large number of ALK-positive lung adenocarcinoma (ADC) patients.We tested 196 lung ADCs for ALK rearrangement by two IHC assays (Ventana pre-diluted ALK D5F3 antibody with the Optiview DAB IHC detection kit and Optiview Amplification kit, D5F3 by Cell Signaling Technology (CST) with Ultraview DAB detection kit by Ventana), fluorescence in situ hybridization (FISH) and real-time reverse transcription–PCR (RT–PCR). CST ALK IHC was scored using the scoring scheme of 0, no staining; 1+, faint; 2+, moderate; and 3+, strong cytoplasmic reactivity in ≥10% of tumor cells. As for Ventana IHC, a binary scoring system (positive or negative for ALK status) was adopted for evaluating the staining results.Among 196 cases tested, 63 (32%), 65 (33%), 70 (36%), and 69 (35%) cases were ALK positive by FISH, Ventana IHC, CST IHC, and RT–PCR, respectively. The sensitivity and specificity of Ventana IHC were 100% and 98%, respectively. Two Ventana IHC-positive cases, which were also CST IHC score of 3+, showed FISH negative, but their ALK rearrangement was confirmed by RT–PCR and direct sequencing. The sensitivity and specificity of CST IHC with staining intensity score of 1+ or more were 100% and 95%, respectively. Five (25%, of 20) patients with CST IHC score of 1+ were both FISH and RT–PCR negative. The sensitivity and specificity of RT–PCR for detection of ALK fusion were 98% and 95%, respectively. The total accordance rate between ALK RT–PCR and Ventana IHC was 97%.The novel fully automated IHC assay is a reliable screening tool in routine pathologic laboratories for identification of patients with ALK rearrangement for targeted therapy in lung ADC.