This vial of blood is the most controversial piece of evidence in the 'Making a Murderer' mystery - here's how the test that was run on it works

The moment viewers think the tide could change in the trial of Steven Avery comes with the discovery of a hole in a tube.

Avery, the subject of the Netflix docuseries "Making a Murderer," is currently serving a life sentence without parole for the murder of Teresa Halbach. But at one point during the series, there's hope that a tube of blood - one that helped exonerate him from an earlier crime - might be the evidence needed to clear his name again.

When the blood sample is recovered during Avery's second trial, it appears as if it has been penetrated by a syringe.

Avery's defense attorneys use that evidence to argue that it had clearly been tampered with - meaning that the blood found in Halbach's car, which was later used to incriminate Avery, could have been planted there.

All the defense needed to prove was that the blood from the tube was the same blood that was found in Halbach's car. And that hinged on one critical chemical: EDTA.

What is EDTA?

EDTA, short for ethylenediaminetetraacetic acid, is a fairly standard part of blood collection. It is used not only as a way to keep blood from clumping together, but also to treat extreme cases of lead poisoning in a process called "chelation therapy."

Business Insider chatted with FRND CEO Coley Parry, whose company does routine blood draws, so they could demonstrate a standard blood collection procedure. EDTA is on the lining of the tube where it mixes in the blood to keep it from coagulating.

The collection tube

What's not disputed in the Avery trial that the tube was indeed punctured - in fact, the nurse responsible for the blood draw was scheduled to testify about how the hole got there.

What is disputed is why the puncture left such a big mark.

To collect blood, FRND uses vacuum-sealed, "vacutainer" tubes - the same kind used to collect Avery's blood. To get the blood into the tube, a health care professional inserts one needle into a vein and another into the vacuum tube. The vacuum in the tube pulls in the blood from needle to needle until it's full. The needle in the tube is then removed and a seal is created in the rubber that was punctured microscopically.

I had my blood drawn by a FRND phlebotomist in December. After my blood was drawn (pictured above), I didn't see any marks on the tube. I called Parry over Skype to have him re-demonstrate the process using water instead of blood. After the draw, he shows me the top of the tube: No marks whatsoever.

According to Parry, there should never be a mark on the top of a tube. And Parry, who's been following Avery's case after watching "Making a Murderer," has his own theory as to how the mark got there. He thinks it's from dried blood which collected outside the stopper. He says dried blood wouldn't have gotten there while the blood was being collected.

"I've never seen blood on the outside of stoppers," said Parry. That's not to say it's not possible, he said, just "very unlikely." Plus, as revealed on "Making a Murderer," the box containing the tube had also been opened, something that Parry said raised his suspicions further.

So how would dried blood have gotten on the outside of the stopper?

Parry says it's feasible that if someone had opened the blood, used a syringe to make a secondary draw, and then transported that syringe to whatever location he or she wanted (say, a car, to fit the theory), the blood could have collected on the stopper then.

Limitations of the EDTA test

Because the blood had been treated with EDTA, it seems like a simple solution to just test for the chemical in the blood at the scene of the crime to determine if it was planted. But, as viewers saw, this wasn't the case.

Rarely do scientists test blood for the presence of EDTA. Usually, it's only used to collect blood. In the Avery trial, a test is designed to look for EDTA which is based on a 1997 study that was then updated for "technology advancements." What exactly those modifications were is not yet public.

The test shows no evidence of EDTA in the blood found in Halbach's car, which looks bad for Avery.

Still, there are some unresolved problems with the test itself. A scientific investigator whom the defense later calls to testify speaks to one of these, saying that a finding of no EDTA can mean one of two things: either there was indeed no EDTA, or the test wasn't very good.

Parry also said he's skeptical of the speed with which the test was assembled. He's probably among many viewers when he says he hopes for a "legitimate test" in the future.