SNOMED CT constraint syntax - for querying instances or terminology?

Thomas Beale thomas.beale at
Tue Sep 29 09:10:42 EDT 2015

The latest version of the SNOMED CT constraint language 
<>proposes expressions like the 

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 
<>, 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 

- thomas
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