The California Department of Insurance opened an investigation into Aetna after a doctor formerly employed by the insurer made an admission under oath: He never looked at patients’ records before deciding to approve or deny care as a medical director.

The investigation, reported by CNN, was opened after former Southern California medical director Dr. Jay Ken Iinuma said he relied on information from nurses when deciding to approve or deny a case — rather than looking at the records himself.

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California Insurance Commissioner Dave Jones told CNN that his office is looking into how common the practice of defaulting to nurses’ reviews and recommendations is within Aetna, the third-largest insurance supplier in the U.S.

“If the health insurer is making decisions to deny coverage without a physician actually ever reviewing medical records, that’s of significant concern to me as insurance commissioner in California — and potentially a violation of law,” he said.

The California Department of Insurance could not be reached for comment Sunday evening.

The testimony came as part of a lawsuit filed against Aetna by a college student, Gillen Washington, who says he was denied coverage for a procedure that would have helped his rare immune disorder. The case is expected to go to trial this week in California Superior Court, according to CNN.

In a statement Sunday night, Aetna said it had yet to hear from the state commissioner but looks forward to “explaining our clinical review process.”

“Aetna medical directors are trained to review all available medical information — including medical records — to make an informed decision,” the company added.

Trisha Thadani is a San Francisco Chronicle staff writer. Email: tthadani@sfchronicle.com Twitter: @TrishaThadani