External Quality Assessment of Immunocytochemistry on Cytology Samples: A 10-Year Analysis of the United Kingdom National External Quality Assessment Scheme for Immunocytochemistry and In-Situ Hybridisation Cytopathology Module.

Abstract

Immunocytochemistry (ICC) is central to diagnostic cytopathology, yet achieving consistent staining quality remains challenging owing to technical complexity and limited standardization. External quality assessment (EQA) programs provide an objective framework for benchmarking ICC performance across laboratories. To analyze performance trends, technical variables, and participant self-assessment in the United Kingdom National External Quality Assessment Scheme for Immunocytochemistry and In-Situ Hybridisation (UK NEQAS ICC & ISH) cytopathology module during a 10-year period. Data from 41 EQA runs (June 2014-May 2024) involving 140 laboratories were analyzed. ICC quality on UK NEQAS-provided cytology slides and participant in-house control slides was evaluated by expert assessors using a predefined scoring system (range, 0-20; satisfactory ≥13). Expert scores, participant self-assessment, and assessors' comments were reviewed. Of 14 639 slides assessed, 90.3% achieved satisfactory staining quality, with significant improvement over time. Cell blocks showed higher mean scores and fewer suboptimal results than cytospins (15.77 versus 15.19; P < .001). Fixation influenced performance on in-house controls: methanol-fixed preparations had higher mean scores than formalin-fixed samples (16.36 versus 16.05; P = .003) but a higher suboptimal rate, while acetone- and ethanol-fixed samples performed less well. Inappropriate control selection was a major cause of suboptimal results. Participant self-assessment correlated weakly with expert evaluation (r = .30) and tended to overestimate performance. ICC performance in cytology is generally satisfactory and improving. While cell blocks offer the most robust standardization, optimized methanol fixation represents a viable alternative for non-cell block samples. The discordance between participant self-assessment and expert review highlights the continuing value of EQA in identifying quality gaps and guiding improvement.