NullFlavor

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HL7 supports multiple exceptional values, i.e. values other than what is prescribed or allowed, under the umbrella concept of a NullFlavor. This is rather different from the traditional concept of null, in that a NullFlavor can stand in for any required value, and still be valid. In addition, there are several different types of exceptional values, some of which convey information.

The various types are as follows:

• NI no information This is the default exceptional value, and is closest in meaning to more conventional uses of the concept of null.
• OTH other There is a value, but it cannot be expressed using the given data type or value set
  • NINF negative infinity
  • PINF positive infinity
• UNK unknown
  • ASKU asked but unknown the expected source could not provide information
  • NAV temporarily not available
  • NASK not asked
  • QS sufficient quantity an unknown quantity was used to achieve a specific goal
  • TRC trace an unmeasurable but nonzero value
• MSK masked there is a known proper value, but it cannot be released in the given context
• NA not applicable

There is also a value for NP (not present) but this is never applicable within a clinical information system, only in messages.