AMCA provides billing collection services to Optum360, which is a Quest contractor. AMCA and Optum360 did not immediately respond to a request for comment from CNN Business.
Quest Diagnostics says almost 12 million of its patients may have had their personal, financial and medical information breached.
On Monday, the United States clinical laboratory said that American Medical Collection Agency (AMCA), a billing collections provider that works with Quest, informed the company that an unauthorized user had managed to obtain access to AMCA systems.
According to the company, someone had unauthorized access to their systems between last August and this past March.
"Quest is taking this matter very seriously" and has "suspended sending collection requests to AMCA", the company said in a press release about the incident, though they admitted they were "not able to verify the accuracy of the information received from AMCA".More news: Trump in the United Kingdom : day one in three minutes
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But AMCA hasn't yet provided it with "detailed or complete" information about the securiy breach, including which information may have been affected, it said.
Quest and Optum360 are working with forensic experts to investigate the matter.
Quest serves half the hospitals and physicians in the United States, and one in three adult healthcare customers, according to its website. Patient laboratory test results were not impacted by this incident, Quest Diagnostics said.
- Quest Diagnostics, one of the nation's biggest clinical laboratories, warned that almost 12 million patients might be affected by a data breach that happened to a collections agency the clinic uses.