Measuring what future physicians know about contraception: development and validation of the clinical contraceptive knowledge test (CCKT) in Vietnam using item response theory

Background Accurate measurement of contraceptive knowledge among medical students remains a critical challenge due to limitations of traditional assessment tools. This study aimed to develop and validate the Clinical Contraceptive Knowledge Test (CCKT) using a modern psychometric framework. Methods A cross-sectional multicenter study was conducted among undergraduate medical students in Vietnam (723 recruited; 636 retained for analysis). The instrument was developed through four sequential phases: (1) Item generation based on Bloom’s taxonomy and WHO Medical Eligibility Criteria (MEC), (2) expert review for content validity, (3) pilot testing using Classical Test Theory, and (4) large-scale validation using Item Response Theory. A two-parameter logistic (2PL) IRT model was applied to evaluate item characteristics, dimensionality, and measurement precision. Results The final instrument, CCKT-18, demonstrated strong psychometric performance. A unidimensional 2PL model was retained based on comparable fit to multidimensional models and high inter-factor correlations (r > 0.80). Model fit indices indicated good fit (CFI = 0.983, TLI = 0.980, RMSEA = 0.036, SRMR = 0.039). The scale showed high measurement precision across a broad range of ability levels, with peak information around the average proficiency level (marginal reliability = 0.844). Item parameters were stable in sensitivity analyses (ΔCFI = 0.003), supporting model robustness. DIF analyses supported measurement invariance across sex, although several items demonstrated DIF across the training stage. Conclusion The CCKT-18 is a valid, reliable, and robust instrument for assessing contraceptive knowledge among medical students. Its IRT-based framework enables precise, competency-oriented evaluation and provides actionable insights for targeted curriculum improvement in reproductive health education.