V2 Lengths

Background

• long controversy concerning lengths
  • most implementers do not take care of length at all, i.e. no change of application with other length definitions!
• lack of clarity concerning what we are trying to do
• problem of the back door
  • introduced by local requirements
  • max length must be exceeded in certain circumstances
  • normative max length is defacto useless

Solution

1. stop reporting lengths on complex types

• value is increasingly difficult to determine
• in an application only "atomic" information is stored
• (value is specific to vertical bar representation)
• value is useless and misleading
• only define lengths for primitive types (ST etc)
• only report lengths were the context of use has different rules to definition of type

2. differentiate between authoritative lengths and arbitrary lengths

• some lengths are authoritative
  • e.g.:
    • for codes coming from HL7-defined tables (they are a MUST)
    • control information
    • segment/field references
    • within realms: a set of fixed values is required in realm specific messages
  • authoritative lengths are fixed and normative - may not be violated
  • systems must support valid data that they encounter
• some lengths are arbitrary - any particular length is wrong
  • e.g.:
    • lengths of local codes and names, address parts
  • Do not make normative rules. But some length guidance is still useful
  • so provide a "conformance length": the minimum length that must be supported
    • any higher length is accepted, but not shorter
  • conformance length offers a degree of predictability without preventing real world problems being solved

3. add support for truncation pattern

• applications currenly truncates data, that is too long
  • in most cases without any notification/recognition
• it works better if systems are able to communicate about truncation
• truncation allows this.
• truncation pattern is not required
  • it is a new optional (5th) character in MSH-2
• some content (identification, clinical descriptions) are not allowed to be truncated
  • this is controlled by a special indication in C.LEN
• some data type specific truncation rules (numbers, dates)

4. make vendors aware of conformance statements/profiles

• documentation is the overall and optimal solution
• whatever a vendor has done, i.e. each individual use of lengths, must be documented
• try to make conformance statements required for conforming applications