Validation of the Idylla <scp> <i>KRAS</i> </scp> Mutation Test on Fine‐Needle Aspiration Needle Rinses in Saline and Cytolyt Solution Without <scp>DNA</scp> Extraction: A Comparison With Paired <scp>FFPE</scp> Tumor Samples

Molecular testing on FNA rinses offers a simple, minimally invasive, and cost-effective alternative to tissue biopsies. Among clinically relevant biomarkers, KRAS mutations are key in guiding targeted therapy across several cancer types. The fully automated Idylla platform provides rapid analysis directly from FFPE samples, without DNA extraction. In this study, we assessed the feasibility of applying the Idylla KRAS Mutation Test to FNA needle rinses collected in saline and Cytolyt. We analyzed 30 FNA rinses from resection specimens of 27 colorectal adenocarcinomas and three pancreatic adenocarcinomas (simulated FNAs). All 30 rinses were collected in saline, and 18 were additionally collected in Cytolyt without Preservcyt transfer. After cytospin preparation to assess cellularity and tumor percentage, the rinse was centrifuged at 2500 rpm for 5 min to obtain a pellet. A 50 μL aliquot of the pellet was directly loaded into the Idylla cartridge. Results from all rinses were compared with the Idylla KRAS Mutation Test on the corresponding FFPE tumor tissue. KRAS mutations were consistently detected in FNA rinses, with no false negatives or false positives observed. A single case, a saline rinse, was misclassified by the automated variant caller (reported as Q61K instead of the true G12D). However, amplification curve review confirmed a clear G12D, while the Q61K signal was aberrant. The Idylla KRAS Mutation Test can be reliably applied to FNA needle rinses in both saline and Cytolyt, yielding robust results in samples with high tumor content. Nevertheless, careful review of amplification curve patterns is essential in this setting.