When a payer needs to notify an EHR of an event (PA decision rendered, member transferred, eligibility changed), two FHIR-native mechanisms apply. CDS Hooks is a workflow-triggered pattern: the EHR calls the payer service at a specific decision point, and the payer responds with information. FHIR Subscriptions is an event-driven pattern: the EHR registers interest in resource changes, and the payer pushes notifications when events happen. Both are conformant; the choice has operational consequences. This comparison lays out the trade-offs for more on payer-side workflow integration on this site.
What Each Pattern Does
CDS Hooks is a request-response pattern. The EHR initiates contact at a defined workflow trigger (order-sign, order-select, appointment-book, medication-prescribe). The payer service receives the call, processes the context, and returns a card with information. The EHR renders the card inline in the workflow. The flow is bounded by the trigger; nothing happens between triggers.
FHIR Subscriptions is a publish-subscribe pattern. The EHR or another consumer registers a Subscription with the payer's FHIR server, specifying which resource changes to be notified about. The payer pushes notifications when matching changes occur. The flow is continuous; events propagate as they happen.
Where CDS Hooks Wins
CDS Hooks wins for clinician-facing in-workflow notification. The EHR has the workflow context; the hook fires at the right moment; the response renders inside the workflow the clinician is already using. For PA pre-checks at order entry, this is the right pattern.
CDS Hooks also wins when the notification is action-driven. The clinician is making a decision; the payer's information needs to arrive at the moment of the decision. Notifications that arrive between decisions are less useful in clinician workflows.
Where FHIR Subscriptions Wins
FHIR Subscriptions wins for status-change notifications that happen outside the clinician's current workflow. A PA decision rendered by the payer's UM team three hours after submission is a status change; CDS Hooks cannot push this back because the EHR is not actively asking. Subscriptions can push.
Subscriptions also win for back-office and care-management workflows where the consumer is not the clinician at the moment of decision. A care management team monitoring a population for PA outcomes wants Subscription-style notification rather than relying on clinician-triggered hooks.
The Patterns Combine
In production, most CMS-0057-F EHR-payer integrations use both patterns together. CDS Hooks handles the initial CRD prompt at order-sign. The clinician launches DTR, completes the form, submits PAS. The PA submission creates a Task that the payer's UM system processes. When the UM system renders the decision, a FHIR Subscription fires that notifies the EHR. The EHR consumes the notification and updates the relevant view (the patient's chart, the order status, the clinician's task list).
This combined flow uses CDS Hooks for the synchronous decision support and Subscriptions for the asynchronous status update. Neither pattern alone covers the full lifecycle.
The EHR Support Asymmetry
EHR support for CDS Hooks is broader and more mature than support for FHIR Subscriptions in 2026. Epic, Cerner Oracle Health, and athenahealth all support CDS Hooks well. FHIR Subscriptions support is patchier; Epic and Cerner have working implementations but smaller EHRs may not.
This asymmetry means CDS Hooks works as a baseline integration pattern across more EHRs, while Subscriptions works only where the EHR supports it. For payers whose provider network spans many EHRs, the practical pattern is CDS Hooks for the synchronous layer and Subscriptions where possible plus polling-based fallback where it is not.
The Performance and Reliability Differences
CDS Hooks calls are synchronous, which means latency and error handling matter at the moment of the clinician's workflow. A slow or unreliable hook degrades the workflow experience. Subscriptions are asynchronous, which means latency matters less but reliable delivery matters more. A dropped Subscription notification means the consumer never knows the event happened.
Production implementations handle both: CDS Hooks with strict latency SLAs and retries on transient failures, Subscriptions with delivery guarantees and consumer-side reconciliation.
How to Read Vendor Positioning
For the CDS Hooks side of EHR support specifically, the Top 5 EHRs with strong CDS Hooks support for Da Vinci CRD covers the EHR-side landscape. For the broader Patient Access API integration patterns that may use Subscriptions for status changes, the Top 5 EHR integration patterns for Patient Access API consumers covers the related layer.