CMS has created a special website that lists the lab CPT codes where, for a hospital outpatient origin specimen, the performing lab must bill, rather than the hospital. The instructions conform to codes that were, and remain, OPPS "Status A" which means that up til now, the hospital was responsible for billing but the codes weren't bundled to a visit or procedure.
CMS is ignoring some misstatements in its November final outpatient rulemaking, such as saying the new billing principle didn't apply to genomic sequencing procedure codes [82 FR 52536 sic]. CMS said that, but didn't mean that, and is applying the DOS exception to Status A mopath codes. These are human molecular pathology codes; microbial molecular codes are not part of this exception.
Find the CMS webpage here; download a zip file from it. The zip file includes an Excel spreadsheet of Status A OPPS Lab Codes, and a PDF with a paragraph of explanation.
The billing authority is obligatory. Up until January 1, the hospital was obligated to bill Medicare (whether the test was performed at-hospital or at a referral lab). This year, the performing lab is obligated to bill (whether it be the hospital or an outside lab.)
The rule is difficult for some referring labs because they often don't know if the specimen is from an inpatient, hospital outpatient, or just an incoming sample to the hospital (not a patient of the hospital on the day). The rule also creates a new date of service, which is the "date the test is performed." In rulemaking, CMS declined to define this date further, and, when asked, declined to formally define it as "the date of the final report," [82 FR 52538] leaving implementation to the laboratory.