In the HCUP databases, ICD-9-CM external cause of injury codes (E codes) are represented as 4- to 5-character alphanumeric codes with implicit decimals (i.e., decimals not included). Prior to data year 2014, the HCUP data elements for E Codes are length 5; in 2014, they are length 7. The first digit is the character E with all subsequent digits being numeric. The codes are left-justified in the HCUP databases so that prior to 2014 there is one space following a 4-character E code (three spaces in 2014). For example, the E code E916 would appear as 'E916 ' with a trailing blank in HCUP data and the code E917.1 would appear as 'E9171'.

The storage of E codes in the HCUP databases varies by data year:

• Beginning in October 2015, International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) external cause of morbidity codes are stored in the data element I10_ECAUSEn.
• From January 2003 through September 2015, any separately reported E codes and any ICD-9-CM E codes encountered in the diagnosis array are placed in a separate array specific to E codes (ECODEn).
• Prior to 2003, ICD-9-CM E codes are included in the diagnosis array (DXn).

E codes are compared to a list of ICD-9-CM codes valid for the discharge date. Anticipation of or lags in response to official ICD-9-CM coding changes are permitted for discharges occurring within a window of time around the official ICD-9-CM coding changes (usually October 1). In the data prior to 1998, a six months window (three months before and three months after) is allowed. Beginning in the 1998 data, a year window (six months before and six months after) is allowed. Invalid and inconsistent E codes are masked directly. Clinical Classifications Software (CCS) data elements are coded with respect to the E code.

Beginning in data year 2009, hospitals in Arizona are required to report E codes for an injury diagnosis in the range 800-999. In addition, the number of E codes that could be retained on a discharge record increased.

Prior to data year 2010, California did not require the reporting of E-codes in the range E870-E879 (misadventures and abnormal reactions).

Starting in 2003, the Georgia Hospital Association requires that any ICD-9-CM diagnosis codes indicating medical misadventures (E870-E879) and adverse reactions (E940-E949) be excluded from the list of diagnosis codes on the record.

Incomplete External Cause of Injury code (E code) information in 2005 and 2006
In 2005 and 2006, the Oklahoma inpatient data provided to HCUP did not include their field for a separately collected External Cause of Injury code (E code). Some E codes were reported with the diagnosis, but the exclusion of this data field means that the E code reporting is incomplete.

Oklahoma requires E codes to be reported for only the initial treatment of the injury. They are not required for transfers or any secondary treatments. E codes are required whenever there is any diagnosis (primary, secondary) of an injury, poisoning, or adverse effect (ICD-9-CM codes 800-999) and it is the initial treatment for that condition.

South Carolina does not provide to HCUP E-codes in the range E870-E876 (misadventures and abnormal reactions).

West Virginia hospitals were not required to submit E-codes. Thus, these fields were almost 99% missing in the 2007 files.