Postcoordination in depth: reading and building complex clusters

Reading a cluster like a sentence

A cluster reads like a sentence once you know the rules. WHO gives five.

One stem, then its extensions

Rule one: if only one stem code is coded, no clustering is needed. Rule two: stem codes are always coded before extension codes. Rule three: one stem with its extension codes uses the ampersand.

• Rule 1 · BA41.0 · One stem code alone needs no clustering mechanism.
• Rule 2 · Stems first · When postcoordinating, stem codes are always coded before extension codes.
• Rule 3 · & · One stem with one or more extension codes: the ampersand.

Anatomy added to a stem

WHO's example. Acute ST elevation myocardial infarction, with two specific anatomy extensions: the anterior wall of the heart, and the left anterior descending coronary artery. Stem first, then each extension, joined with ampersands.

• BA41.0 · Stem · Acute ST elevation myocardial infarction.
• XA7RE3 · Anatomy · Anterior wall of heart.
• XA7NQ7 · Anatomy · Left anterior descending coronary artery.

Two stems: order by use case

Rule four: when two stem codes are linked, order follows the use case, and a slash separates them. For mortality, if only one code can be kept, the aetiology comes first. For morbidity, the main condition comes first.

Each stem keeps its extensions

Rule five: when extensions belong to different stem codes, the syntax must make the ownership clear. Each stem is followed by its own extensions with ampersands, and a slash separates the parts.

• First part · DD51&XK8G · Inguinal hernia, left.
• Second part · ME24.2&XT5R · Digestive system obstruction, acute.

Cluster one concept, not two

One boundary. Postcoordination is only for combining codes that fully describe a single documented concept. Two distinct concepts, with separate stem codes, are not reported together. WHO's pedestrian injury example reports a concussion and a fracture as two clusters.

Check yourself

Pause and think. Two stem codes are linked and only one can be kept for mortality. Which comes first? The answer: The aetiology. Rule four: for mortality, if only one code can be kept, the aetiology comes first. For morbidity, the main condition comes first.

• The main condition
• The aetiology
• The manifestation
• Whichever was recorded first

Keep these in mind

Stem first, then extensions. An ampersand for extensions and a slash between stems. The order of stems follows the use case. And one concept per cluster.

• Stem first, then extensions
• Ampersand for extensions, slash between stems
• Order of stems follows the use case
• One concept per cluster