Green CDA Over the Wire

Over the past decade, I've been closely involved in the evolution of clinical summary/transition of care content standards. We started with CDA, then merged the CDA and CCR to create the CCD, which we further constrained with the C32 implementation guide. This year, the Consolidated CDA initiative refined the CCD/C32 specifications from IHE, HITSP and HL7 into one easy to use implementation guide.

The only problem with CDA-based standards is that implementation requires expert knowledge of the HL7 Reference Information Model (RIM), a steep learning curve for new developers. Ideally, HL7 and informatics experts would use the RIM or Detailed Clinical Models to develop templates containing easy to read and parse XML that does not require knowledge of the underlying information model.

Green CDA aims to do that - the simplicity of CCR XML tagging with the expandability and modeling of CDA. However, there is a controversy.  What do we send on the wire - full CDA or Green CDA?

At the November HIT Standards Committee meeting we endorsed moving forward with GreenCDA as the single over the wire format.  

It's likely that existing users of full CDA over the wire will be concerned about the costs and effort required to move to Green CDA on the wire.

Robert Warden, an expert on HL7 mapping tools, wrote a great summary that explains the path forward through the use of transforms:

"There is now tooling which can define a Green CDA and generate 100% reliable transforms to full CDA, all in the same step. That tooling is being used in the UK for the NHS Interoperability Toolkit; and it has been linked to MDHT, so it could be used to Green consolidated CDA and generate the transforms.

Once you have reliable, maintainable transforms, the question “Green CDA or full CDA on the wire?” becomes much less crucial. Any organization, such as ONC, which wishes to endorse Green CDA over the wire (to make life much easier for implementers), can do so, provided they make the transforms readily available. Then, people can use Green CDA over the wire – but anyone who prefers to receive full CDA can always do so, by applying the transform to the Green CDA he receives. RIM experts and non-experts can both be happy.

It is like using banknotes instead of gold for currency, as has been done for several hundred years. Using gold bars for every transaction would be very clumsy, so people used notes which were backed by gold. The bank “promises to pay the bearer on demand” the equivalent in gold. Similarly the Green=>full transform “promises to pay the bearer on demand” a full CDA, if he wants it.

HL7 has now realized that its CDA gold bars are rather clunky, and should encourage the use of banknotes, backed by its RIM-based gold semantics. CDA is the reserve currency, and Green CDAs are the banknotes that people want to use (but counterfeit notes, not backed by the gold transforms, would be worthless!)."

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