Introduction The current level of SDM in daily practice remains below expectations, necessitating an evaluation. Current methods of measuring SDM have their limitations, particularly the patient-reported metrics, which are susceptible to misinterpretation and bias. Thus, there is a need for a tool that measures patients‘ knowledge and comprehension of the SDM concept before evaluating their perceived level of SDM. This study developed and validated a questionnaire to assess the current level of SDM knowledge among participants and to identify individual characteristics related to SDM knowledge.

Methods We adhered to established guidelines for questionnaire development while integrating conceptual models for assessing and structuring knowledge of SDM. The questionnaire was tested and refined through expert reviews, pilot tests, and validation studies involving 210 participants at Amsterdam University Medical Centers. Statistical analyses included item analysis, Cronbach’s alpha for internal consistency, the Rasch model plus hypothesis testing.

Results The 25-item Shared Decision-Making Knowledge Questionnaire (SDM-K-Q) demonstrated good validity and reliability (Cronbach’s alpha = 0.74 and Rasch model acceptance). The mean SDM knowledge level among participants was 73.7%. Predictors of higher SDM knowledge included higher educational levels, healthcare occupation, and consistent appointment preparation, while having a migration background was a negative predictor.

Discussion The questionnaire proved to be an innovative, reliable tool, effective in measuring SDM knowledge as single construct, and demonstrated the ability to differentiate between specific groups. Consistency was based on a single administration, so further validation efforts should solidify effectiveness and establish cut-off values to classify SDM knowledge levels.

Conclusion The SDM-K-Q represents a significant advancement in understanding patients‘ knowledge of SDM. It can help identify knowledge gaps and evaluate the impact of educational interventions. Further validation and utilization of the SDM-K-Q can assist in developing SDM implementation strategies.