Kurt Cobain was found dead with triple the lethal dose of morphine

Men ... I'm gonna give something else to think about ... Everywhere it says that Kurt Cobain had triple the lethal dose of drugs in his body ...
Now comes the big surprise ... How can anyone said something like that if on the "Kurt"'s body was no autopsy ? There is a know fact that there were no autopsy on Kurt Cobain ... So how can they say the body had 3 times the lethal dose ?
I'm not a heroin addict but I can see an overdose ... But how can you say for sure it was 3 times the lethal dose ?