I pulled this article. The last paragraph really sums it up. It depends on if they get admitted to inpatient following the ER visit. If I'm reading correctly, "It's the patient's chief complaint in the ER but if they get admitted it's then the principal diagnosis"
Some payers have encouraged, or insisted, that hospitals report the presenting symptoms for emergency room visit, even when a definitive diagnosis is established and reporting the symptoms would violate the official coding guideline concerning the reporting of symptoms integral to the definitive diagnosis. The payers are requesting this information in order to establish the emergent nature of the patient's complaint. The presenting symptoms (such as chest pain) may justify an emergency room visit, but the definitive diagnosis (such as hiatal hernia) is a non-urgent condition and would not, by itself, justify a trip to the emergency room.
To solve this dilemma while maintaining data integrity and adhering to official coding guidelines, the National Uniform Billing Committee agreed to expand the title and definition of the admitting diagnosis field on the UB-92 claim form to accommodate the need for information regarding the presenting sign or symptom. The title of this data element has been expanded to include "patient's reason for visit." The definition has been modified to read: "the ICD-9-CM diagnosis code describing the patient's diagnosis or reason for visit at the time of admission or outpatient registration." For outpatient claims, this should be the ICD-9-CM code describing the patient's stated reason for seeking care (or as stated by the patient's representative, such as parent, legal guardian, or paramedic). The modification of the description and definition of the Admitting Diagnosis field met several objectives, including:
- facilitating claims processing by allowing providers to report the reason the patient presented for treatment
- the new outpatient definition is consistent with the intent of the prudent layperson legislation that seeks to establish the reason the patient is seeking care (which may differ from the diagnosis established by the physician at the conclusion of the visit)
- the outpatient definition is consistent with various national definitions for the patient's reason for visit
- providing explanation as to why certain tests may have been ordered and performed
- reducing administrative burden on providers and payers by eliminating requests for additional documentation in some cases
- promoting adherence to established national coding guidelines
An ICD-9-CM diagnosis code should be reported in the admitting diagnosis field on the UB-92 whenever there is an unscheduled outpatient visit to a healthcare facility's emergency room or urgent care center. Currently, the UB-92 claim form can only accommodate one diagnosis code to describe the patient's primary reason for seeking care or treatment. The diagnosis code describing the patient's reason for the unscheduled visit should only be reported on outpatient claims. If the unscheduled visit results in an inpatient admission, the admitting diagnosis code should be reported instead of the reason for the outpatient visit. The use of the admitting diagnosis field for outpatient emergent and urgent encounters became effective April 1, 2000, and applies to all payers