A UnitedHealth Group Inc. unit wrongly denied coverage for a teenager’s extended stay at an inpatient residential treatment center for mental health and substance abuse issues, the Tenth Circuit ruled Tuesday.

The insurer determined that the teenager’s care wasn’t medically necessary to treat his mental health conditions without meaningfully considering whether it was necessary to treat his substance abuse issues, the US Court of Appeals for the Tenth Circuit said.

The doctor who denied the family’s internal appeal “tacitly rejected” substance abuse as an independent ground for coverage by failing to mention it, the court said.

The record is …