The Diagnosis Master Category List (DMCL), created by the National Center for Health Statistics (NCHS), 
Division of Health Care Statistics, is a classification scheme that can be used across inpatient and 
ambulatory settings when analyzing data. The DMCL is based on the International Classification of 
Diseases, 10th Revision, Clinical Modification (ICD-10-CM). There are approximately 70,000 ICD-10-CM 
codes, which necessitates the creation of a smaller number of diagnosis categories that can be used in 
health care research for presenting clinically meaningful estimates of health data. 

The intent of the DMCL is to include categories appropriate for use across both inpatient and ambulatory 
settings. Thus, it includes categories that reflect diagnosis items collected for the various National 
Health Care Surveys, incorporates information from older diagnosis classification schemes based on the 
International Classification of Diseases, 9th Revision, Clinical Modification (ICD-9-CM), and includes 
categories that may be used in publications and online web tables. Additionally, the DMCL contains new 
categories, adjusts to the change from ICD-9-CM to ICD-10-CM, and also accounts for any annual changes 
to the ICD-10-CM.  

The process of creating the DMCL involved bimonthly meetings with a DHCS workgroup, consisting of subject 
matter experts including medical systems specialists, physicians, statisticians, health services 
researchers, and programmers. Initially, a combined list of diagnosis categories was created taking 
categories from older diagnosis classification schemes (see links above). Then, the workgroup discussed 
each diagnosis category to determine whether it was still appropriate to include. The workgroup further 
discussed the possibility of adding new diagnosis categories based on research relevance and interest. 
Finally, diagnosis items collected on the patient record form (PRF) of NAMCS/NHAMCS were also included. 
After the first draft of the DMCL was finalized, two medical systems specialists assigned ICD-10-CM codes 
corresponding to each category. After the two medical systems specialists came to agreement on code 
assignments, analysis was conducted by applying the first draft of the DMCL scheme to hospital and ambulatory 
health care data and comparing frequencies across all settings. Categories with low or high counts across all 
settings were discussed by the workgroup; for low count categories, removing the categories was discussed; 
and for high count categories, breaking them out into more specific groups within the category was discussed. 

The DMCL uses all ICD-10-CM codes and classifies them into mutually exclusive categories. The DMCL will 
slightly change each year to account for annual changes to the ICD-10-CM coding system which can result in a 
different number of categories each year. The number of categories each year are: 323 in 2017, 341 in 
2018-2019, and 343 in 2020-2022.

Starting in 2018, codes from the chapter “Injury, poisoning and certain other consequences of external causes” 
and codes from the categories “Land transport accidents” and “Other transport accidents” were truncated to 6 
characters if the 7th character was a letter representing initial, subsequent, sequela, or other type of 
encounter. For example, the codes S100XXA, S100XXD, and S100XXS were truncated to S100XX.