Post-coordination: build a cluster

The problem

A clinician writes: acute pyelonephritis, left side, E. coli. One stem code names the condition, but the side and the organism are clinical detail too, and they need somewhere to go.

Three building blocks

• A stem code is the diagnosis and is valid on its own.
• An extension code adds detail, such as severity, side, anatomy or cause. It can never stand alone and can never come first.
• A cluster is the group of codes that belong together.

Build it

Start with the stem code for the condition. Then add each extension, one at a time, joined with an ampersand: the side, then the organism. The result is one cluster that carries the whole diagnostic statement.

Two joiners

An ampersand (&) joins a stem code to its extension codes. A forward slash (/) joins two stem codes, such as an ulcer and the bleeding it caused. When two stem codes are combined, their order follows the use case, for example mortality or morbidity.

The most common mistake

Acute RSV bronchiolitis already has its own code, with the virus built in. Adding an unspecified stem code and an extension code rebuilds that meaning in pieces. WHO says post-coordination must never replicate a pre-coordinated code.

Recap

Read the stem, add extensions in order, use the right joiner, and never rebuild what a single code already says.