An estimated 11.9 million patients’ personal and medical information may have been exposed in a data breach of a collections agency that works with Quest Diagnostics Inc. and the insurer UnitedHealth Group Inc.

Quest said in a securities filing that it had been informed of the breach by American Medical Collection Agency, an Elmsford, New York-based collections firm. For eight months, an unauthorized user had access to personal information including credit card numbers and bank accounts, medical information, and personal information such as Social Security numbers.

Quest, which operates medical testing centers around the U.S., said it has suspended sending...