Implement a CDS Hooks patient-view service that uses a prefetch template to retrieve Observation resources without a server-side FHIR call and returns a gap-in-care card when a required screening is overdue

Steps

1. In your CDS service discovery document, define a prefetch object on the patient-view hook service entry with a key such as recentObservations whose value is a FHIR search URL template using the token placeholder for the patient context parameter, scoped to LOINC codes representing the screenings you are tracking
2. In the hook handler, check whether the prefetch key is populated in the request body; if the EHR honored the prefetch template, the value will be a FHIR Bundle; if it is null or absent, make a direct FHIR server call to the fhirServer base URL provided in the hook request using the fhirAuthorization token
3. Evaluate each returned Observation's effective date against the recommended screening interval for the patient's age and risk category; compute whether the most recent result falls within the required window
4. If a gap is detected, construct a CDS Hooks card with indicator info and a detailed summary, and include a link action with a type of absolute pointing to the clinical guideline or smart form URL for ordering the overdue screening
5. Attach a source object referencing the clinical guideline that defines the screening interval, such as a USPSTF recommendation, so the clinician can distinguish EHR-vendor cards from guideline-backed alerts
6. Return an empty suggestions array and no system actions if the screening is current so that the EHR does not display a card and alarm fatigue is minimized