HL7 Flavors of Null / Reasons Why Data is Missing

code name definition
NI No information No information whatsoever can be inferred from this exceptional value. This is the most general exceptional value. It is also the default exceptional value.
OTH other The actual value is not an element in the value domain of a variable. (e.g., concept not provided by required code system).
UNK unknown A proper value is applicable, but not known
ASKU asked but unknown Information was sought but not found (e.g., patient was asked but didn’t know)
NAV temporarily unavailable Information is not available at this time but it is expected that it will be available later.
NASK not asked This information has not been sought (e.g., patient was not asked)
MSK masked There is information on this item available but it has not been provided by the sender due to security, privacy or other reasons. There may be an alternate mechanism for gaining access to this information.Note: using this null flavor does provide information that may be a breach of confidentiality, even though no detail data is provided. Its primary purpose is for those circumstances where it is necessary to inform the receiver that the information does exist without providing any detail.
NA not applicable No proper value is applicable in this context (e.g., last menstrual period for a male).
NP not present Value is not present in a message. This is only defined in messages, never in application data! All values not present in the message must be replaced by the applicable default, or no-information (NI) as the default of all defaults.

Section 1.11.4 of HL7 Data Type Specification

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