NDX indicates the total number of ICD-9-CM diagnoses (valid and invalid) coded on the discharge record. In assigning NDX, the first listed diagnosis is included in the count, even if it is blank, so long as there is a secondary diagnosis present (see table below).

For data beginning in the fourth quarter of 2015, the count of the number of diagnoses is stored in the data element I10_NDX to indicate the implementation of the ICD-10-CM/PCS coding system.

A maximum of 15 diagnoses has been retained on a NIS inpatient record. States that provide fewer than 15 diagnoses have had the diagnosis vector padded with blank values. For example, if a state supplied 5 diagnoses, DX6 through DX15 are blank (" ") on all records from that state. States that provide more than 15 diagnoses may have information truncated. If an inpatient record from these states had more than 15 non-missing diagnoses, diagnoses in positions 16 and above were not included in the NIS file. If the number of diagnoses coded on this discharge (NDX) is greater than 15, secondary diagnoses have been truncated from the record.

Refer to the general note for the diagnosis fields (DXn) for the number of diagnoses provided in each state.

Since on the NIS the number of diagnoses coded on the discharge (NDX) can be greater than the number of diagnoses available on the inpatient record, caution needs to be taken when using NDX to loop through the diagnoses. A counter for the loop should not extend past 15. Programming code such as the following example SAS statement is needed to take this into account:

DO I = 1 to MIN(15,NDX);
Followed by code to process all diagnoses.
END;