What is the difference between selection of the principal diagnosis for inpatients versus outpatients?

The most important difference is that the Uniform Hospital Discharge Data Set (UHDDS) definition of principal diagnosis were developed for inpatient reporting and do not apply to the outpatient. Because diagnoses are not established at the time of the initial outpatient encounter or visit, this is an extremely important guideline. In many outpatient cases, the diagnosis code for a presenting sign or symptom must be assigned because a definitive diagnosis has not yet been determined.