(CBS Local) —  One of the biggest blood testing providers in the country is warning nearly 12 million customers their personal data may have been breached. Quest Diagnostics released a statement Monday saying their billing collections service provider, American Medical Collection Agency (AMCA), notified them that an unauthorized user had access to AMCA’s system containing personal information AMCA received from various companies, including Quest.

AMCA provides billing collections services to Optum360, which is a Quest contractor.

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AMCA says the information breached includes personal information, including certain financial data, social security numbers, and medical information, but not laboratory test results.

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The company first notified Quest and Optum360 on May 14 of potential unauthorized activity on AMCA’s web management page. On May 31, they were notified that the date on AMCA’s affected system included information regarding approximately 11.9 million Quest patients.

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