I'm not sure, but let's say that's true. They usually also don't care to know the limits. Another interesting case is Patricia Stallings (emphasis mine):

an American woman who was wrongfully convicted of murder after the death of her son Ryan on September 7, 1989. Because testing seemed to indicate an elevated level of ethylene glycol in Ryan's blood, authorities suspected antifreeze poisoning, and arrested Stallings the next day. She was convicted of murder in early 1991, and sentenced to life in prison.

Stallings gave birth to another child while incarcerated awaiting trial; this next child was diagnosed with methylmalonic acidemia (MMA), a rare genetic disorder that can mimic antifreeze poisoning. Prosecutors initially did not believe that the sibling's diagnosis had anything to do with Ryan's case. Stallings' lawyer was forbidden from producing available evidence as proof of the possibility. After a professor in biochemistry and molecular biology had some of Ryan's blood samples tested, he was able to prove that the child had also died from MMA, and not from ethylene glycol poisoning.