In Medicare claims processing, CMS processes laboratory CPT codes against edits related to CLIA lab status. Most lab codes can only be processed for an entity with full CLIA certification, but dozens are "CLIA WAIVED" and this is flagged by added "QW" as a two-place modifier.
On December 23, CMS released some updated QW additions, specifically to the rapid antigen code 87811 and to the multi-antigen code (influenza + Covid) 87428.
Find the CMS source document here:
CMS also has a master list of all new and old QW codes online here. For a MAC webpage with notes on how to use QW, here. For the full text of R10529OTN, including instructions to MACs on what an EUA is, see OTN10529 here. OTN means "one time notification."
As occurs with other CMS transmittals, the dates can be quirky. This one is released December 23, effective October 6, but needs to be implemented (by MAC computers) only by April 5, 2021.
To skim recent CMS announcements, here.