Objectives
To assess the content validity and psychometric characteristics of the Knowledge about Older Patients Quiz (KOP‐Q), which measures nurses' knowledge regarding older hospitalized adults and their certainty regarding this knowledge.

Design
Cross‐sectional.

Setting
Content validity: general hospitals. Psychometric characteristics: nursing school and general hospitals in the Netherlands.

Participants
Content validity: 12 nurse specialists in geriatrics. Psychometric characteristics: 107 first‐year and 78 final‐year bachelor of nursing students, 148 registered nurses, and 20 nurse specialists in geriatrics.

Measurements
Content validity: The nurse specialists rated each item of the initial KOP‐Q (52 items) on relevance. Ratings were used to calculate Item‐Content Validity Index and average Scale‐Content Validity Index (S‐CVI/ave) scores. Items with insufficient content validity were removed. Psychometric characteristics: Ratings of students, nurses, and nurse specialists were used to test for different item functioning (DIF) and unidimensionality before item characteristics (discrimination and difficulty) were examined using Item Response Theory. Finally, norm references were calculated and nomological validity was assessed.

Results
Content validity: Forty‐three items remained after assessing content validity (S‐CVI/ave = 0.90). Psychometric characteristics: Of the 43 items, two demonstrating ceiling effects and 11 distorting ability estimates (DIF) were subsequently excluded. Item characteristics were assessed for the remaining 30 items, all of which demonstrated good discrimination and difficulty parameters. Knowledge was positively correlated with certainty about this knowledge.

Conclusion
The final 30‐item KOP‐Q is a valid, psychometrically sound, comprehensive instrument that can be used to assess the knowledge of nursing students, hospital nurses, and nurse specialists in geriatrics regarding older hospitalized adults. It can identify knowledge and certainty deficits for research purposes or serve as a tool in educational or quality improvement programs.