Beginning in the fourth quarter of 2015, International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) diagnoses reported on HCUP records are stored in the data element I10_DXn. In data prior to the fourth quarter of 2015, International Classification of Diseases, Ninth Revision, Clinical Modification (ICD-9-CM) diagnoses reported on HCUP records are stored in the data element 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-10-CM diagnoses are represented by alphanumeric codes with a maximum length of 7 characters and implicit decimals (i.e., decimals not included). The HCUP data elements for ICD-10-CM diagnoses are length 7. The first digit is always character, the second digit is always numeric, and all subsequent digits can be character or numeric. The codes are left-justified in the HCUP databases. For example, the diagnosis code C4A.4 would appear as 'C4A4 ' with trailing blanks in the HCUP data.

The original value of the first listed diagnosis (I10_DX1), whether blank or coded, is retained in the first position of the diagnosis vector. Starting at the first secondary diagnosis (I10_DX2), the diagnoses are shifted during HCUP processing to eliminate blank secondary diagnoses. For example, if I10_DX2 and I10_DX4 contain nonmissing diagnoses and I10_DX3 is blank, then the value of I10_DX4 is shifted into I10_DX3. Secondary diagnoses are never shifted into the first listed position (I10_DX1).

Beginning with the 2017 HCUP databases, external cause of morbidity codes are included with other diagnoses in the diagnosis-related data elements (I10_DXn, I10_NDX, and DXPOAn). These external cause of morbidity codes were retained in separate data elements (I10_ECAUSEn, I10_NECAUSE, and E_POAn) for quarter four of 2015 and all of 2016. Under ICD-9-CM (prior to quarter four of 2015), the separate external cause of injury data elements were retained in the data elements ECODEn, NECODE, and E_POAn.

Diagnoses are compared to a list of ICD-10-CM codes valid for the discharge date. Anticipation of or lags in response to official ICD-10-CM coding changes are permitted for discharges occurring within a window of time (six months before and six months after) around the official ICD-10-CM coding changes (usually October 1). 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) and the patient's age (edit checks EAGE04 and EAGE05) for checking the internal consistency of the record.

Invalid and inconsistent codes are handled in the following way:

The maximum number of ICD-10-CM diagnoses reported varies by state. HCUP retains all diagnosis fields provided by the data source.