Wiki Paring of principal diagnosis with surgery

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If a patient is admitted with abdominal pain and respiratory symptoms after careful study the patient the patient is diagnosed with pneumonia and acute appendicitis and has an appendectomy. Is there a rule that states you must use the principal diagnosis that is associated with the surgery? Or can you code pneumonia as the prinicpal diagnosis and use the other codes to code the surgery?
Your principle dx should be the presenting problem. I assume your surgeon was consulted or admitted the patient for the abdominal pain which turned out to be acute appendicitis. The appendicitis should be your primary dx with the pneumonia being secondary as this would be considered a co-morbidity. You should always use the "reason" for the visit/surgery as the primary dx.