2c389a02__Specifications___openEHR_wiki__openEHR_equivalent
[0] 3 [Code mnemonic] ASKU [Rubric] asked but unknown [Description] Information was sought but not found (e.g., patient was asked but didn't know) [openEHR equivalent] 
In openEHR, this is a legitimate response of the patient, and is not a case of missing data. See discussion above. I really don't understand that comment; the patient responded that they didn't know, so the data is missing
Null Flavours and Boolean data in openEHR - Specifications - openEHR wiki
openEHR equivalent