Background <p>The c-ros oncogene 1 (ROS1) rearrangement is one of the actionable targets in non-small cell lung carcinoma (NSCLC), particularly the adenocarcinoma subtype. Early detection of this mutation is crucial for determining appropriate therapy. While Reverse Transcriptase Polymerase Chain Reaction (RT-PCR) remains the gold standard for ROS1 rearrangement detection, immunohistochemistry (IHC) is often used as a screening method due to its accessibility and rapid processing time.</p> Aim <p>This study evaluates the sensitivity and specificity of IHC compared to RT-PCR in detecting ROS1 rearrangement.</p> Materials and methods <p>This was a diagnostic cross-sectional study conducted at H. Adam Malik Hospital and Santa Elisabeth Hospital in Medan between December 2022 and December 2023. A total of 90 patients with NSCLC adenocarcinoma who met the inclusion and exclusion criteria were enrolled. Collected data included clinical characteristics and results of ROS1 rearrangement testing using RT-PCR and IHC. The validity of IHC was assessed by calculating sensitivity, specificity, positive predictive value (PPV), negative predictive value (NPV), and diagnostic accuracy, using RT-PCR as the reference method.</p> Results <p>Most patients were male (66.7%), with a mean age of 56.46 years, and the majority were former smokers (67.8%). Among the 90 patients, 8 (8.9%) were ROS1-positive by RT-PCR and 7 (7.8%) were ROS1-positive by IHC. The sensitivity of IHC compared to RT-PCR was 37.5%, specificity was 95.1%, PPV was 42.9%, and NPV was 94.0%. The overall diagnostic accuracy of IHC was 90.0%.</p> Conclusion <p>Immunohistochemistry shows high specificity and negative predictive value, making it a reasonably effective screening tool for ROS1 rearrangement detection. However, its low sensitivity and positive predictive value indicate that IHC should not be used as a standalone diagnostic method for determining ROS1 status.</p>