Is it acceptable to populate the claim level CAS04 with a zero (0) when there is no change to the covered days?
This issue is explicitly addressed in guide 005010X221. The situational rule on the 2100 loop CAS segment element 04 (Adjustment Quantity) reads "Required when the CAS02 adjustment reason code is related to non-covered days. If not required by this implementation guide, do not send."
If there is no change to covered days (CAS04 = 0), then the adjustment reason code in CAS02 is not related to non-covered days, and the guide instruction is "do not send".
Therefore, population of the 2100 loop CAS04 with a zero is not compliant with the guide instructions.