This variable shows the date when the beneficiary first met the criteria for the Chronic Condition Data Warehouse (CCW) Alzheimer's disease and related disorders or senile dementia indicator. The variable will be blank for beneficiaries that have never had the condition.
The earliest possible date for anyone in the CCW is January 1, 1999. If the beneficiary became eligible for Medicare after that, the earliest possible date will be some time after his/her coverage start date (i.e., the COVSTART variable in the Beneficiary File).
Source: CCW (derived)
From CY 2017 to CY 2021, the MBSF: Chronic Conditions segment also has a newer (CC30) version. The algorithm used to create the First Occurrence of Alzheimer's Disease and Related Disorders or Senile Dementia in the CC30 version includes different DX codes, a reduced reference period, and an increased claim requirement. Different DX codes means that codes were added and removed between versions.