FHIR - openEHR Data Types cross-analysis

Introduction

As part of its 'fresh look' activity, Grahame Grieve at Health Intersections has produced a set of simplified data types that solves a lot of the problems of ISO 21090, and addresses many of the needs of openEHR data types. It may become a new specification within HL7 or elsewhere, and is a candidate data types model for CIMI. These data types are part of a larger piece of work called Fast Health Information Resources (FHIR). This work is gathering pace in HL7, so it seems a worthwhile point to consider its possible impact on openEHR.

There are two types of analysis we can do on the FHIR data types. The obvious one is to do with information representation needs, i.e. a gap analysis of the FHIR data types versus the openEHR data types. This is a relatively detailed analysis to carry out, and will take some time. It is needed to determine how to convert FHIR-based data (e.g. HL7 messages of the future) in and out of openEHR data.

The second kind of analysis is to do with archetyping. We ask the question: could FHIR data types satisfy the requirements met by the current openEHR data types (DV_CODED_TEXT, DV_QUANTITY, and friends), remembering that archetyping needs are generally a subset of information persistence and communication needs. To give a simple example: the data type DV_QUANTITY in openEHR has a precision attribute, but not one of the 250+ CKM archetypes mentions it. This analysis is easier to perform, and its outcome enables us to know if FHIR data types could be used for archetyping purposes.

Analysis

The tables below present a gap analysis of openEHR v FHIR data types, seen both from the perspectives of information representation and archetype needs. In the tables below, statistics generated from the CKM archetypes as at 06 Jun 2011 are provided in the second column. Obviously things change, but these numbers are understood to be broadly representative.

Primitive types

Of all of the available primitive types defined in the openEHR Support IM specification, only a few are relevant to archetyping.

openEHR does not define primitive types as such. But since such types are needed ubiquitously in specifications, it provides a set of partial class definitions that it assumes. These definitions support the openEHR specifications. For example, there is a String type, and in the specifications, some kind of 'empty' function is often needed. So the openEHR Support IM assumed_types package defines String and a few functions, including is_empty() on it.

openEHR data types	No. times constrained in CKM *	FHIR data types	Archetype Requirements	Information Representation Requirements
Octet	0	-
Character	0	-
-	-	base64binary
Boolean	62	boolean	-
Integer	50	integer (32 bit) decimal (integer values >32 bit)	Integer appears in the DV_COUNT openEHR type. Currently there is no way (and as far as we know, no demand) to constrain an integer field to be of a certain size such as 64 bit or 32 bit.	The current assumption as to the size of 'Integer' in an openEHR system is that a) the size is decided by the implementation and that b) normal conversion rules would apply to enable typical integer values (i.e. relatively small numbers) stored in 64-bit words to be correctly converted to 32-bit or smaller integers.
Real	57	-	Real is used in DV_QUANTITY and is the type of most lab and many other quantities.
String	7	string	Strings are rarely directly constrained other than for force them to be non-empty, since any particular constraint might be language specific.	In FHIR, all atomic identifiers (Oids, UUIDs etc) are considered subtypes of String, with special constraints on the string pattern.
ISO8601_DATE	3	humanDate
ISO8601_DATE_TIME	5	datetime
ISO8601_TIME	0	-
ISO8601_DURATION	13	see DV_DURATION below.
Array<T>	-	-
List<T>	-	-
Set<T>	-	-
Hash<T,U>	-	-
Interval<T>	-	-

Id types

openEHR data types	FHIR data types	Comments
OBJECT_REF	-	xx
OBJECT_ID	-	xx
INTERNET_ID	-	xx
UUID	uuid	xx
ISO_OID	oid	xx
LOCATABLE_REF	-	xx
TERMINOLOGY_ID	sid	In openEHR, strings of the form 'name(ver)' are used, where the 'ver' part is optional. In FHIR, the sid primitive type appears to have values from the left column of this table.
ARCHETYPE_ID	-	xx
OBJECT_VERSION_ID	-	xx
HIER_OBJECT_ID	-	xx
GENERIC_ID	Identifier

Complex Types

openEHR data types	No. times constrained in CKM archetypes *	FHIR data types	Archetyping	Data Representation	Feedback to FHIR
DV_BOOLEAN	56	-	Used widely in archetypes.		No FHIR subtype of DataType to accommodate booleans?

DV_ORDERED .normal_status .normal_range .other_reference_ranges
DV_QUANTIFIED .magnitude_status		Quantity .status
DV_AMOUNT .accuracy .accuracy_is_percent
DV_QUANTITY .magnitude .units .precision	189 133 161 66	Quantity .value .units	Used widely in archetypes.	In FHIR, Quantity accommodates both Integer and Real values. There is an optional field status that matches openEHR DV_QUANTITY.magnitude_status, but has fewer values.	The FHIR choice to represent units as a code or a string seems unnecessarily complicated. A UCUM units string should be adequate. There is an example with unit=mcg/L and code=ug. What can this mean?
DV_COUNT .magnitude	28	Quantity
DV_PROPORTION .numerator .denominator	40	Ratio
DV_ORDINAL .value .symbol	50	Choice			The FHIR type does not seem to accommodate values, only symbols. Test for FHIR: how to represent Apgar or Barthel ordinal value lists?
DV_INTERVAL<T:DV_QUANTIFIED>	5 - DV_DATE 1 - DV_COUNT 1 - DV_QUANTITY 1 - DV_DATE_TIME	Interval

DV_DATE	25	xx	xx
DV_DATE_TIME	32	xx	xx
DV_TIME	0	xx	xx
DV_DURATION	22	Quantity	xx

DV_TEXT .value .mappings .language .encoding .hyperlink .formatting	521	CodeableConcept .text .coding (repeats)	DV_TEXT mostly used in archetyping to simply indicate the field type.		In the FHIR type, the value of the 'text' and the 'displayName' fields will be the same. Given the ubiquity of coded data elements in health data, this seems unfortunate. No way to know what language the text is in?
DV_CODED_TEXT .defining_code	388	CodeableConcept .primary	Numerous fields in archetypes are constrained to be local codes, and in templates, to external codes or refsets.		''

DV_URI	6	uri	xx
DV_EHR_URI	2	uri	xx
DV_PARSABLE	1	-	xx
DV_IDENTIFIER .issuer .id .assigner .type	2	HumanId .identifier.system .identifier.id .assigner .type .period		This should be a near perfect bidirectional transform apart from the FHIR HumanId 'period' attribute.	We would consider this a risk, because a validity period might be assigned initially, then be changed; should the data be changed? Can this field be reliably set and reasoned about computationally?

DV_ENCAPSULATED .charset .language
DV_MULTIMEDIA .alternate_text .uri .media_type .data .compression_algorithm .integrity_check .integrity_check_algorithm .size	13	Attachment .title .url .mimeType .data [Not compressed] .hash [Std Hash] [derivable]		The FHIR type appears to correspond to allow only base64 ASCII-encoded binary, as is done with MIME attachments in email. This alleviates the need for the 'compression' attribute used in DV_MULTIMEDIA. Any openEHR content should be easy to convert to FHIR format; assuming an openEHR system makes sensible local decisions about binary representation of large multimedia objects, Attachment => DV_MULTIMEDIA should also be trouble-free.
DV_TIME_SPECIFICATION .value: syntax expression with: &period &calendar_alignment &event_alignment &occurrences &institution_specified	n/a	Schedule - .duration/.frequency .event .when .count -	n/a	These openEHR types are taken from HL7, and so far have limited use in openEHR systems. It would appear reasonable to replace the openEHR type with whatever HL7 decides on for this type in the future, which might be this FHIR type.
DV_GENERAL_TIME_SPECIFICATION .calendar_alignment .event_alignment .institution_specified	0	Schedule		''
DV_PERIODIC_TIME_SPECIFICATION	0	Schedule		''
-		Money