One of the key statements of the CCR and CCD specifications is found in the very first sentence of the CCR standard (emphasis is mine):
1. ScopeThis statement answers the oft-asked question: In a CCD/CCR, which observations, medications, et cetera, should be sent? The answer is "the most relevant" or "pertient" ones. The notion of relevance, and the need for it in the standard is best shown in John Halamka's post on Standards for Personal Health Records. What providers don't want is a pile of "electronic paper" to wade through. The terms relevant and pertinent were terms that were chosen with great care and deliberation by the members of the ASTM Heatlhcare Informatics committee for that very reason.
1.1 The Continuity of Care Record (CCR) is a core data set of the most relevant administrative, demographic, and clinical information facts about a patient’s healthcare, covering one or more healthcare encounters. It provides a means for one healthcare practitioner, system, or setting to aggregate all of the pertinent data about a patient and forward it to another practitioner, system, or setting to support the continuity of car
I get asked the question about what to send quite a bit in the HL7 class on the Continuity of Care Document (which I'll be teaching again in Cambridge, MA in October at the HL7 Working Group meeting), and in other venues. The answer I give is always the same. Relevance is a clinical decision that right now needs to be made by a trained care provider, and is based on context. What is relevant to one care provider is not necessarily relevant to another. For example, while your dentist may want to know that you've experienced a heart attack, details such as which side of the heart, or the exact ejection fraction ratio, may not be as relevant to them as they would be to your cardiologist or GP.
The question of relevance depends not only on who you are communicating with, but also what is known at the time of communication. An observation that may not be relevant today could be of importance later once new medical knowledge is discovered. Obviously, you can only determine relevance based on what you know right now. Trying to cover ones anterior anatomy by transmitting data that "might be relevant in the future, but isn't known to be relevant now" is just another way of trying to avoid making a clinical decision.
A lot of people are struggling with how to determine relevance without the immediate judgment of a care provider. They want to know what the definition of relevance is. There are no simple answers here. This is a clinical decision support problem, and while there are many different guidelines, there are no hard and fast rules. Even a rule such as "Send the first, send the worst, and send the last" migh err in sending either too much, or too little. The worst blood pressure reading for a patient might be during surgery, when it expectedly dropped as a result of the procedure. Is that the "right" blood pressure result to send? Probably not. Which is worse? A white blood cell count that is very high or very low? It depends, and it may well be that what you want is the trend over the last week because the patient is recuperating from an infection after chemotherapy. Again, context is important, and I suspect that any attempt to set a rule of thumb will fail.
What I suggest developers pay attention to is the user interface that providers need to select the most relevant clinical observations to send. Once you have some experience with that, and with what providers actually do with it, you can start thinking about how to provide more clinical decision support for providers.
Another question I get asked quite a bit is when to send a CCD/CCR, and is usually in the context of long inpatient stays. My answer for that is fairly simple, at transfers of care. I've also been asked whether a transfer from emergency to inpatient counts as a transfer of care. This is also pretty simple, in the US it is a policy question, and the relevant policy makers say yes.
P.S. IANAL, but I strongly suggest that algorithms determining the most relevant data to show a healthcare provider are properly in the domain of FDA regulated medical devices.