This field must:
- start with a digit or a Z
- be reported as per coding guidelines

For the provision of Diagnosis Related Groups, some ICD diagnosis codes cannot be used as a Principal diagnosis, and will result in a fatal error in the grouping process.

Diagnosis codes starting with a V or Y, describing the circumstances that cause an injury, rather than the nature of an injury, cannot be used as a Principal diagnosis.  

Diagnosis codes starting with an M are morphology codes, and cannot be used as a Principal diagnosis and will result in a fatal error.

A Principal diagnosis should be recorded for each episode of patient care.  Where the principal diagnosis is recorded prior to discharge (as in annual census of public psychiatric hospital inpatients), it is the current provisional principal diagnosis.  Only use the admission diagnosis when no other diagnostic information is available.  The current provisional diagnosis may be the same as the admission diagnosis.