Medicare Part B medical insurance helps pay for some services and products not covered by Part A (hospital insurance) for Americans aged 65 and older who have worked and paid into the system. It also provides health insurance to younger people with disabilities.

Part B coverage begins once a patient meets his or her deductible ($147 in 2013), then typically Medicare covers 80% of approved services, while the remaining 20% is paid by the patient, either directly or indirectly by private Medigap insurance.

For clinical labs and pathology groups, Part B covers laboratory and diagnostic tests. Laboratory tests include certain blood tests, urinalysis, tests on tissue specimens, and some screening tests. They must be provided by a laboratory that meets Medicare requirements.

Complex rules are used to manage the benefit, and advisories are periodically issued which describe coverage criteria. On the national level these advisories are issued by CMS, and are known as National Coverage Determinations (NCD). Local Coverage Determinations (LCD) apply within the multi-state area managed by a specific regional Medicare Part B contractor, and Local Medical Review Policies (LMRP) were superseded by LCDs in 2003.

Medicare Part B payments make up about 15% of the revenue of the two biggest national lab companies. By contrast, it is common for community labs to have between 30% and 65% of their revenue come from Medicare Part B payments.

Part B coverage can also be provided by private insurers through Medicare Advantage Plans. Enrollment in private Medicare Advantage plans has more than doubled since 2006, according to the New York Times. As these plans gain popularity, clinical labs and pathology groups continue to find themselves without access to patients they once served. Medicare beneficiaries now enrolled in Advantage plans comprise nearly one-third of all Medicare beneficiaries.

Generally speaking, growth in Medicare Advantage enrollment favors the national labs, with private insurers providing them exclusive network contracts. This means less market access to these patients by community labs.