The inpatient principle diagnosis can only be achieved after the patient has been examined and studied to ascertain the main cause of the condition.
For example, a person who is complaining of chest pain might be suffering from pneumonia or a heart condition. The symptom: chest pain can be the primary diagnosis (PDX) for admittance to the hospital. The principle diagnosis will only occur after the specific cause is found after careful study of the patient. In this case it was pneumonia.
Some lessons I have learned to make me a better coder is don't assume anything, if I have to, I must query the physician to find out