The latest version of the SNOMED CT constraint language <http://snomed.org/expressionconstraint>proposes expressions like the following:

Section 6.3
....
To find those capsules that have a strength between 500 and 800 mg (inclusive), the following expression constraint may be used:

<  27658006 |amoxicillin| :
    411116001 |has  dose form| = <<  385049006 | capsule|,
    { 111115 |has basis of strength| =  ( 111115 |amoxicillin only| :
111115 |strength magnitude| >= #500, 111115 |strength magnitude| <= #800,
        111115 |strength unit| =  258684004 |mg|)}

The purpose of this is apparently as a query to find instances of Amoxycillin recorded somewhere - presumably in EHRs, or some kind of prescriptions database? However, the document talks of executing queries over a 'Substrate', defined as 'The SNOMED CT content over which an expression constraint is evaluated or a query is executed.'.

Elsewhere (p12) it says:

Please note that the substrate over which the expression constraint is evaluated is not explicitly defined within the expression constraint, and must therefore be established by some other means. By default, the assumed substrate is the set of active components from the snapshot release (in distribution normal form) of the SNOMED CT versioned edition currently loaded into the given tool.

It is also not clear what the query really means - is it a query for Amoxycillin that has been prescribed for a specific patient? For any patient in some cohort? Or for a medication order that has been suspended, postponed, or completed?

More generally, I am not clear how the one language is intended to be used across SNOMED CT itself (e.g. to generate intentional ref sets) and also across instance data. If it's the former, it can only be concept-based; if the latter, the query won't correspond to whatever information model is in use for the data.

How would it be used with data based on (for example) the openEHR medication item archetype in the UK clinical models CKM <http://www.clinicalmodels.org.uk/ckm/>, shown below?




The recommendations in section 7.5 for executing the queries seem to imply that they are for evaluation against SNOMED CT.

There are various other oddities, but I'll leave them for later.

if anyone who is working with this can clarify, it would be most appreciated.

- thomas
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