Story highlights There were an estimated 33,000 opioid-related deaths in the US in 2015, officials say

But new research suggests that even more uncounted deaths may have occurred

(CNN) Experts say the United States is in the throes of an opioid abuse epidemic, causing 91 overdose deaths each day. Yet the total number of opioid-related deaths may still be underestimated, suggests new research from the US Centers for Disease Control and Prevention.

"In early spring, the Minnesota Department of Health was notified of an unexplained death: a middle-aged man who died suddenly at home," said Dr. Victoria Hall, a CDC field officer based in Minnesota. He'd been on long-term opioid therapy for back pain, and his family had worried he might be abusing his medication. The medical examiner assigned to the autopsy tested for and diagnosed both pneumonia and a toxic level of opioids.

"However, on the death certificate, it only listed the pneumonia and made no mention of opioids," Hall said.

The researchers say it may be difficult to track causes of death, such as this one, within surveillance systems that are based solely on autopsy report codes known as International Classification of Diseases, Tenth Edition, or ICD-10.

Over half of the deaths involving opioids in her study had not been captured in the state's total, said Hall.