Transitioning Laboratory Examinations Beyond Their Originally Intended Scope: A Conceptual Framework Illustrated by Digital Microscopy of Vaginal Suspensions

Background: Medical laboratories may use examination procedures outside their originally intended scope, including a new specimen matrix, an extended measurement range, a different reagent–instrument combination, or a modified decision rule. These changes require a controlled transition before routine use. Objective: This concept paper proposes a stage-gated framework for transitioning a laboratory examination beyond its originally intended scope. Digital microscopy of vaginal suspensions for binary classification of Trichomonas vaginalis infection and Candida albicans-associated vulvovaginal candidiasis (VVC) is used only as an illustrative case. Methods: The framework was informed by Regulation (EU) 2017/746, MDCG guidance, ISO 15189, CLSI EP12, and secondary diagnostic literature. No patient specimens, digital images, instrument-generated results, hardware changes, or software modifications were used. Literature-derived sensitivity and specificity ranges were modeled with bounded triangular distributions using 100,000 Monte Carlo iterations in NCSS 2026 (64 bit), version 26.0.5. Results are reported as scenario intervals, not as confidence intervals or estimates of an actual device. Results: The framework comprises six gates: change classification, intended-purpose definition, technical feasibility, performance and risk evidence, implementation decision, and controlled use with monitoring. In the illustrative scenarios, median sensitivity/specificity were 56.7%/96.5% for T. vaginalis and 50.3%/94.5% for C. albicans-associated VVC. Predictive values changed materially across prevalence scenarios, demonstrating why validation criteria must be linked to the intended-use population. Conclusions: The proposed framework supports planning, governance, and transparent separation of secondary-scenario modeling from empirical performance assessment. Prospective analytical and clinical validation with real specimens remains mandatory before implementation.