---
title: Message Linkage
slug: epa-accelerator/guide/message-linkage
docTags:
createdAt: 2026-08-12T18:25:29.946Z
---
Message linkage requirements and best practices are described in the table below. Four key fields are used to link messages together, as defined below. This linkage allows a software system to tie one message to another. For example, on a PAInitiationResponse the PBM/payer system needs to find the PAInitiationRequest to which the provider vendor is referring. This linkage should be done in an automated manner.
:::hint{type="info"}
NCPDP refers to message linkage as trace number.
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**MessageID** – The unique number assigned to each message. This ID initiates the electronic linking of subsequent message types.
**RelatesToMessageID** – The MessageID of the most recent and relevant message in the workflow.
**PAReferenceID** – Assigned by the prescribing system on the initial message (or on PARequest message for Prior Authorization renewal requests) and is used by the prescribing system as a tracking identifier on all request and response prior authorization messages. The identifier must be unique per prescribing system for at least 18 months.
**PACaseID** – PBM/payer-assigned identifier for this request and used by the PBM/payer as a tracking identifier on all request and response prior authorization messages.
# PAInitiationRequest and Response Fields
| PAInitiationRequest | PAInitiationResponse | PARequest | PAResponse |
| ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------ | ------------------------ | ------------------------ |
| MessageID = 123 | MessageID = ABC | MessageID = 456 | MessageID = DEF |
| RelatesToMessageID:
If Eligibility is used, RelatesToMessageID contains the Interchange Control Number (ISA13 of Eligibility Response (271)).
If RTPB is used, then RelatesToMessageID contains the RTPB Response MessageID.
If RxChange is used, RelatesToMessageID contains the RxChangeRequest MessageID.
**Note**: If multiple products are used, use the latest MessageID that was used as a trigger for the PA. | RelatesToMessageID = 123 | RelatesToMessageID = ABC | RelatesToMessageID = 456 |
| PAReferenceID = XYZ | PAReferenceID = XYZ | PAReferenceID = XYZ | PAReferenceID = XYZ |
| PACaseID = N/A | PACaseID = 999 | PACaseID = 999 | PACaseID = 999 |
| Status | Status | Status | Status |
| MessageID = 111 | MessageID = 222 | MessageID = 333 | MessageID = 444 |
| RelatesToMessageID = 123 | RelatesToMessageID = ABC | RelatesToMessageID = 456 | RelatesToMessageID = DEF |
# PA Appeal and Cancel Request and Response
| PA Appeal Request | PA Appeal Response | PA Cancel Request | PA Cancel Response |
| ------------------------ | ------------------------ | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | ------------------------ |
| MessageID = 789 | MessageID = GHI | MessageID = 012 | MessageID = JKL |
| RelatesToMessageID = DEF | RelatesToMessageID = 789 | RelatesToMessageID relates to the most recent message received from your trading partner. See definition above.
Since PA Cancel Requests can be sent at any time after receiving case ID, this would relate to the most recent message.
For example, if this was sent after provider sent the PARequest (and had not yet received a PAResponse from the PBM/payer) the RelatesTo ID would be ABC. | RelatesToMessageID = 012 |
| PAReferenceID = XYZ | PAReferenceID = XYZ | PAReferenceID = XYZ | PAReferenceID = XYZ |
| PACaseID = 999 | PACaseID = 999 | PACaseID = 999 | PACaseID = 999 |
| Status | Status | Status | Status |
| MessageID = 555 | MessageID = 666 | MessageID = 777 | MessageID = 888 |
| RelatesToMessageID = 789 | RelatesToMessageID = GHI | RelatesToMessageID = 012 | RelatesToMessageID = JKL |
# Third-Party Processor Message Linking
When PBM/payers return a CO or CP reason code, the message becomes marked as closed for third-party processing. If Surescripts’ third-party processor search finds a match, Surescripts creates a secondary PAInitiationRequest based upon the initial PAInitiationRequest. The recipient is updated and a secondary MessageID with a prefix of EPAINI is sent to the third-party processor. When the third-party processor responds, the message has a RelatesToMessageID that has a prefix of EPAINI and references the secondary PAInitiationRequest.
:::hint{type="info"}
If the PBM/payer referenced is not connected to Surescripts for electronic prior authorization but the third-party processor is, Surescripts forwards the message to the third-party processor as if they were the PBM/payer – no modifications to the MessageID are made.
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## PAInitiationRequest and Response from Third-Party Processor
| PAInitiationRequest (To EPAINI) | PAInitiationResponse (From PBM/Payer - Closed for Third Party) | Second PAInitiationRequest (To Third Party) | Second PAInitiationResponse (From Third Party)\*\* | PARequest (To Third Party) | PAResponse (From Third Party |
| ------------------------------------------------------------------------------------------------- | -------------------------------------------------------------- | ------------------------------------------- | -------------------------------------------------- | -------------------------- | ---------------------------- |
| MessageID = 123 | MessageID = ABC | MessageID = EPAINI321 | MessageID = DEF | MessageID = 789 | MessageID = 010 |
| RelatesToMessageID contains the Interchange Control Number (ISA13 of Eligibility Response (271)). | RelatesToMessageID = 123 | RelatesToMessageID = ABC | RelatesToMessageID = EPAINI321 | RelatesToMessageID = DEF | RelatesToMessageID = 789 |
| PAReferenceID = XYZ | PAReferenceID = XYZ | PAReferenceID = XYZ | PAReferenceID = XYZ | PAReferenceID = XYZ | PAReferenceID = XYZ |
| PACaseID = N/A | PACaseID = N/A | PACaseID = N/A | PACaseID = 123456 | PACaseID = 123456 | PACaseID = 123456 |
| Status | Status | Status | Status | Status | Status |
| MessageID = 111 | MessageID = 222 | MessageID = 333 | MessageID = 444 | MessageID = 555 | MessageID = 666 |
| RelatesToMessageID = 123 | RelatesToMessageID = ABC | RelatesToMessageID = EPAINI321 | RelatesToMessageID = DEF | RelatesToMessageID = 789 | RelatesToMessageID = 010 |
:::hint{type="info"}
If Surescripts cannot find a third-party processor, Surescripts will return a Closed PAInitiationResponse with EPAINI in the From field and possibly a link to a web-based PA forms provider.
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