This code system http://hl7.org/fhir/data-absent-reason defines the following codes:
| Lvl | Code | Display | Definition | Comments |
| 1 | unknown | Unknown | The value is not known. | |
| 1 | asked | Asked | The source human does not know the value. | |
| 1 | temp | Temp | There is reason to expect (from the workflow) that the value may become known. | |
| 1 | not-asked | Not Asked | The workflow didn't lead to this value being known. | |
| 1 | masked | Masked | The information is not available due to security, privacy or related reasons. | Using "masked" may be breach of security or confidentiality, but there are times when its use is required to support alternate workflows for gaining access to denied information. |
| 1 | unsupported | Unsupported | The source system wasn't capable of supporting this element. | |
| 1 | astext | As Text | The content of the data is represented in the resource narrative. | it may be linked by internal references (e.g. xml:id). This usually implies that the value could not be represented in the correct format - this may be due to system limitations, or this particular data value |
| 1 | error | Error | Some system or workflow process error means that the information is not available. | |
| 2 | NaN | Not a Number | NaN, standing for not a number, is a numeric data type value representing an undefined or unrepresentable value. | This is sometimes an output value from measuring devices |
| 1 | not-performed | Not Performed | The value is not available because the observation procedure (test, etc.) was not performed. | |