To facilitate use of patient-centered health IT applications in everyday life, a high degree of usability is required. Based on the example of a patient-centered web application, we propose a usability study design enabling developers and researchers to assess usability of patient-centered health IT applications and derive implications for their improvement. Our study design integrates tasks that subjects have to process, an associated questionnaire based on Perceived Ease of Use, Perceived Usefulness, Attitude Toward Using, and Behavioral Intention to Use, a System Usability Scale questionnaire, and focus groups. Application of the usability study design demonstrates its feasibility and provides insights for assessment of usability in related projects in research and practice.