Prince's autopsy, particularly the toxicology report, may clarify whether prescription painkillers played any part in his death.
Determining whether the music superstar died of a drug overdose will likely involve not only tests of his blood, urine, liver tissue and fluid from the eyes, it also will require compiling evidence from Prince's medicine cabinet, his medical history and possibly information from witnesses and those who knew him.
• Prince: His life and music
Authorities haven't said when results of the April 22 autopsy will be released, only that it would take weeks.
What's already known?
Questions about Prince's health surfaced April 15, when his plane made an emergency stop in Moline, Illinois. He was found unconscious aboard the aircraft, a law enforcement official who was briefed on the investigation told The Associated Press. The official spoke on condition of anonymity because he was not authorized to speak to the media.
While the plane was on the tarmac, the official said, first responders gave Prince a shot of Narcan, an antidote used to reverse suspected opioid overdoses. The official said investigators are looking at whether Prince overdosed on the flight and whether an overdose killed him six days later. One possibility is the powerful painkiller Percocet or something similar, the official said.
A medical examiner completed the physical autopsy in four hours on April 22, the day after Prince was found dead at age 57. The Midwest Medical Examiner's Office did not release a preliminary cause of death based on the autopsy alone and has said the toxicology results would take weeks.
Why does it take so long?
Toxicology reports can be based on multiple rounds of tests, escalating in sophistication as pathologists, toxicologists and chemists work together to answer questions, according to the College of American Pathologists.
First, basic screens are run on blood and urine to detect categories such as opiates, amphetamines, marijuana, alcohol and barbiturates. Confirming those results can take days and sometimes requires repeated testing.
More tests are then run to identify each chemical's unique fingerprint by measuring its mass and other traits. Sometimes, a specialty lab is called in to test for specific drugs. Finding multiple drugs in a person's system can add time to the process.
Tests typically are run on various fluids and tissues including bile and the eye.
"The blood only gives you a snapshot of exactly what was happening at the moment of death," said Dr. Bruce Levy, a pathologist at the University of Illinois at Chicago. "By looking at other body fluids or tissues, you can get more of a sense of history of a person's use of a drug or medication."
What could the autopsy show?
The autopsy report will help clarify whether Prince had a medical issue such as a heart problem that might explain his death, said Dr. Paul Wax, executive director of the American College of Medical Toxicology.
"If he didn't have some other medical condition, then was there enough evidence based on the forensic toxicology testing and other information from pill bottles, family, friends and eyewitnesses to suggest that the death was from drugs?" Wax asked.
If prescription drugs are involved, Prince may have been taking medications exactly as directed, or taking more than prescribed. The report could include evidence that prescriptions from several doctors were involved, or may say nothing about the source of any culprit drugs.
Death certificates sometimes include mention of "polysubstance abuse" — abuse of three or more drugs — or "substance abuse" or perhaps "drug use disorder," Wax said. In Minnesota, such terms can be listed as a contributing factor.
What drug levels are lethal?
It's tricky. Anyone who takes prescription opioid painkillers for a long time builds a tolerance to the drugs. A dose that could kill one person might provide medicinal pain relief to another.
"That's the problem with opioids," said Dr. Lewis Nelson, medical toxicologist at New York University School of Medicine. "Unless you have a good sense of how much opioid the person was using in the past several weeks, it's hard to know how to interpret a post-mortem blood level."
Crucial evidence such as pill bottles, Prince's medical history and other information would come into play, Nelson said. "The number has to be contextualized."
Would the brand name Percocet show up on an autopsy report?
Percocet would show up as separate listings for its ingredients — acetaminophen and oxycodone — on a typical forensic toxicology report, Wax said.
Oxycodone is an opioid, but both ingredients can be dangerous. Acetaminophen overdoses kill about 150 people per year, and would show up as liver damage on an autopsy report. Overdose from opioids, including oxycodone and heroin, killed nearly 29,000 people in 2014.
What might not be released publicly?
Minnesota law gives medical examiners broad authority to access a deceased person's medical records, including those on mental health and chemical dependency. The state allows medical examiners to include references to mental health records in a final summary report, but shields the records themselves from public disclosure. That allows a measure of confidentiality.
"I think the autopsy report will be very objective," said Dr. Yashpal Agrawal of the College of American Pathologists. "It will say what the drug levels are, what drugs were found and give an opinion about what it means." He does not expect the autopsy report to make conclusions about addiction or chemical dependency.
Who is the medical examiner?
Dr. A. Quinn Strobl, who has been the chief medical examiner at the Midwest Medical Examiner's Office since late 2009, performed the autopsy on Prince herself. Her office is the official coroner for 19 counties in Minnesota, including Carver County, where he was found dead.
Strobl has been a practicing forensic pathologist since she finished her fellowship in 2005 and is board-certified in anatomic, clinical and forensic pathology.
According to a 2009 Star Tribune article, Strobl is a native of Philadelphia who attended Penn State and the University of Pittsburgh School of Medicine. She considered going into family practice and surgery, and decided being a medical examiner was a good mix of the two.
"I interact directly with the family, I deliver the diagnosis and I answer a wide spectrum of questions," she told the newspaper. "I don't deliver the bad news. Hopefully, I deliver answers."
What happens next?
Autopsy reports usually include a cause of death such as "opioid poisoning" or "congestive heart failure" and a manner of death, such as "homicide," "suicide," "accident" or "undetermined." But those categories have no legal standing, according to Dr. Dave Fowler, Maryland's chief medical examiner and president of the National Association of Medical Examiners.
"Medical definitions largely are there to classify a death for statistical purposes," Fowler said. "It's more of a public health tool than it is a legal or law enforcement tool."
Asked whether a determination of "accidental overdose" would make it more difficult to file any charges, Fowler said: "Medical examiners are not in the position to be judge and jury. The legal decision is made by local prosecutors or grand juries. They have the authority to file charges. Whatever we call it doesn't make any difference to them. It shouldn't."