Question: Binding choices

Jun 21, 2016

Question: A question regarding “Standard” codes vs. Customized codes: To my understanding, if I want to exchange FHIR resource, any property in that resource can be filled by either of the following options:

  • Using the standard clinical coding system for relevant clinical properties (for example, SNOMED CT code for MedicationOrder>Medication> code
  • Using the HL7 FHIR administrative coding system for relevant administrative properties (for example,http://hl7.org/fhir/ValueSet/ medication-order-statusfor MedicationOrder>Status)
  • Using customized, in-house, coding system, but for that I need to define it as appropriate ValueSet, bounded into appropriate Profile.

Answer:

So this answer (and the question, really) applies to any property in a resource that has a type of Coding or CodeableConcept. In principle, any property of a resource that has a type can be filled by any one of those choices - a SNOMED CT code (or RxNorm, or LOINC, or …), a FHIR defined code, or a locally defined code. However for all these properties, we ‘bind’ them to a value set, and that value set makes a rule about what kind of codes can be used.

Element <– (binding) –> ValueSet

So if the property is bound to SNOMED CT, then you have to use a code from SNOMED CT. But note that if the type is CodeableConcept, which can have more than 1 coding, then this means that one of the codings has to come from SNOMED CT - for the others, you have can use anything at all that you like.

Further, the binding itself has an important property - the ‘strength’ of the binding. This key property tells you how to interpret the binding:

required You have to use one of the specified codes
extensible You have to use one of the specified codes, unless there isn’t an appropriate one. Then you can use whatever you like
preferred This is the sort of codes you should use - and this value set really is a good idea to use
example This is the sort of codes you should use

So the last 2 binding strengths are not, well, binding: you can ignore them as you see fit. So what you can actually use in a resource property depends on the strength of the binding, and the value set it references. But, sadly, most of the really interesting properties in most resources have a binding strength of ‘example’ - precisely because they are most interesting, we have no way to get consensus on the right coding system (let alone the right set of concepts). A great example is Condition.code:

 code   1..1 CodeableConcept Identification of the condition, problem or diagnosisCondition/Problem/Diagnosis Codes(Example)

An example binding to a large set of SNOMED CT codes….

Finally, some notes on your choices:

  • Standard clinical coding system: we always recommend that you use one of these. But these can be hard to use. We introduced theterminology serviceto make this easier, and this is a concept that’s just about in the goto-market phase
  • FHIR code systems are mostly defined for the fixed properties like status where you have to use one of the fixed codes we defined, but sometimes we define a code system to be used with Coding/CodeableConcept. We generally try pretty hard to not overlap with the standard terminologies, so it’s usually a choice of one or the other
  • When you use a customised in house coding system, you don’t actually have to define it using a value set, but doing so allows you to tell everyone else in a computable fashion what you are doing, so it’s a good idea, yes.