Z13.31 on a positive depression screen: write up the correction the practice is waiting on (it earns no CMS2 credit, and 3+ is the PHQ-2 cutoff)

Use the issue details and the owner’s outstanding message as the source for the correction, then record the conclusion beside CMS2 in docs/MEASURES.md. Explain that Z13.31 earns no CMS2 credit, that the 3+ threshold applies to PHQ-2 rather than PHQ-9, and that coded follow-up through the WebChart write path (#565) is the needed alternative. Done means the message is sent and the documentation is updated.

On the 2026-09-10 practice call a design decision was reached out loud and agreed by everyone present: when a depression screen is positive, stamp Z13.31 on the assessment — as a screening code, deliberately not a depression diagnosis, so the patient is not labelled. Someone asked for it to be written up. It never was.

It needs to be written up, because as stated it does not do what the practice believes it does, in two separate ways:

1. Z13.31 earns no CMS2 credit. CMS2's numerator wants a coded follow-up — a Procedure, ServiceRequest or MedicationRequest drawn from the measure's own value sets. A diagnosis code recording that a screen happened is not follow-up, so a patient with a positive screen and a Z13.31 stamp is still numerator-negative. The screening half of the denominator is satisfied by the PHQ result itself, not by the Z code.
2. "3 or higher" is the PHQ-2 cutoff, not the PHQ-9's. The threshold quoted on the call belongs to the two-item screen. Applied to a PHQ-9 it would flag a large number of patients as positive who are not, and the follow-up obligation would then attach to all of them.

The risk is not that we built the wrong thing — we built nothing. It is that the practice left that call believing a decision was made and is being documented, and will build a workflow habit on it. Every week that passes makes the correction more expensive to deliver.

What this issue is: write the correction up and send it (it belongs in the owner's outstanding message to the practice), and record the conclusion in docs/MEASURES.md beside CMS2 so the next person to be asked has the answer. No code.

What would actually satisfy CMS2, and what should be offered instead: the coded follow-up reaching the chart — which is the WebChart write path (#565). Until that exists, the honest answer is that WorkWell can show the gap and cannot close it from its own side.

Found while re-auditing the four pilot source documents on 2026-09-18. Related: #565, #570.