In data prior to the fourth quarter of 2015, diagnoses reported on HCUP records are based on the International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) and stored in the data elements DXn. Beginning in the fourth quarter of 2015, diagnoses reported on HCUP records are based on the International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) and stored in the data elements I10_DXn.

In the HCUP inpatient databases, the first listed diagnosis is the principal diagnosis defined as the condition established after study to be chiefly responsible for occasioning the admission of the patient to the hospital for care. In the HCUP outpatient databases, the first listed diagnosis is the condition, symptom, or problem identified in the medical record to be chiefly responsible for the outpatient visit.

In the HCUP databases, ICD-9-CM diagnoses are represented as 3- to 5-character alphanumeric codes with implicit decimals (i.e., decimals not included). Prior to data year 2014, the HCUP data elements for ICD-9-CM diagnoses are length 5; in 2014, they are length 7 in the HCUP State Databases. . The first digit may be numeric or character (characters E or V only) with all subsequent digits being numeric. The codes are left-justified in the HCUP databases so that prior to 2014 there are two spaces following a 3-character diagnosis code and one space following a 4-character diagnosis code (four and three spaces, respectively, in 2014). For example, the diagnosis code 863.0 would appear as '8630 ' with trailing blanks in HCUP data. Any zeroes at the beginning of the code are significant; they are part of the code. For example, the ICD-9-CM diagnosis code 086.3 would be stored in the HCUP databases as '0863 ' and the diagnosis 008.63 would be stored as '00863'.

The original value of the first listed diagnosis (DX1), whether blank or coded, is retained in the first position of the diagnosis vector. Starting at the first secondary diagnosis (DX2), the diagnoses are shifted during HCUP processing to eliminate blank secondary diagnoses. For example, if DX2 and DX4 contain nonmissing diagnoses and DX3 is blank, then the value of DX4 is shifted into DX3. Secondary diagnoses are never shifted into the first listed position (DX1).

Prior to 2003, external cause of injury codes (E codes) are included in the diagnosis array (DXn). Beginning in 2003, any separately reported E codes and any E codes found in the diagnosis array are retained in a separate array specific to E codes (ECODEn).

Diagnoses 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. If the diagnosis contains intermittent blank characters or is zero filled, then the diagnosis will be considered invalid.

Diagnoses are compared to the sex of the patient (edit check EDX03 beginning in the 1998 data and ED1nn prior to 1998) and the patient's age (edit checks EAGE04 and EAGE05 beginning in the 1998 data and ED3nn and ED4nn prior to 1998) for checking the internal consistency of the record.

How invalid and inconsistent codes are handled varies by data year.

• Beginning in the 1998 data, invalid and inconsistent diagnoses are masked directly. Validity flags are not included on the HCUP record. Clinical Classifications Software (CCS) data elements are coded with respect to the diagnosis.

• Prior to 1998 data, invalid and inconsistent diagnoses are retained on the record. Validity flags (DXVn) indicate invalid, inconsistent diagnosis codes. Clinical Classifications Software (CCS) data elements use the former name (DCCHPRn). The CCS was formerly known as the Clinical Classifications for Health Policy Research (CCHPR). The diagnosis related data elements are coded as follows:

The validity flags (DXVn) need to be used in connection with any analysis of the diagnoses (DXn).

The maximum number of diagnoses reported varies by state. HCUP retains all diagnosis fields provided by the data source.