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Wednesday, December 14, 2011

Integrating Schematron Rules and Clinical Terminology Services

On one of the discussions on the Structured Document Work's CCD mailing list, a member notes that Schematron validation such as that found in the NIST Validator doesn't support validation of content using restricted value sets well.  He's right in that Schematron doesn't include an explicit mechanism designed to perform validation against value sets.  But there is a way to integrate a clinical terminology service into a Schematron rule set using the XSLT document() function and Schematron <let> statement along with appropriately structured rules.  The IHE SVS profile was designed to support this sort of validation, as I mention in a previous post.

For validation, their are two practical classifications of value sets: Those that can be enumerated fully in single XML document, and those which cannot practically be enumerated.  The dividing line is based on the available system memory for the Validator.  I would expect that value sets containing hundreds of elements could practically be enumerated in full, but those containing thousands or more terms would not be practically enumerable.  Note that I do not address issues of "static" vs. "dynamic" value sets here, because the value sets can be enumerated dynamically through a web service call.

The mechanism for validating the smaller value sets in Schematron is to create a variable that contains the content of an XML document.  This is done at the top of the Schematron using the <let> statement:

<sch:let name='ValueSet' value='document("")'/>

Later in the Schematron rule set, you'd have a rule context where you would use that variable as follows:

<sch:rule context='cda:manufacturedMaterial/cda:code'>
  <sch:assert test='$ValueSet//svs:Concept[@code = current()/@code'>
   ... report error if concept is not found ...

This idea was used in CDA Implementation guide Schematrons as far back as 2005, in the Schematron use for the Care Record Summary release 1.0.  In that Schematron, an external file was used (so the URL would have been file:voc.xml), rather than an HTTP URL.

Now, when the value set is large (such as a list of LOINC Lab Results, SNOMED CT problems, or RxNORM Drugs), it's not practical to enumerate every term because the resulting document would be very large.  In these cases, you could enhance the SVS defined Web Service to support a code parameter.  When this parameter was present, the service would return the entry for the single code in that value set when present, or an empty ConceptList if it didn't exist.  The rule context remains the same in this case, but the assertion changes:

  <sch:assert test='document(concat("",@code)//svs:Concept'>
   ... report error if concept is not found ...

In this example, the HTTP Web Service request is dynamically created.  If it returns an empty code list, the rule fails, but if it finds the code, the rule succeeds.

So, while Schematron itself does not support "validation against value sets", appropriate integration with a Clinical Terminology Service and a very simple RESTful API does enable it.  I hope that the NIST Validator takes advantage of this approach in the future.