The Los Angeles Clippers, Atlanta Hawks, New York Knicks and Dallas Mavericks perform blood tests.

Blood tests are used to keep a gauge on the body, revealing levels of stress and fatigue. The test can also tell when a player isn't getting enough sleep, what food is needed and what vitamins and minerals are lacking.

“One of the big upsides is blood don’t lie,” said Shawn Arent, a kinesiology professor at Rutgers University and the director of its Center for Health & Human Performance. “You can ask an athlete how they’re feeling and all that’s important but athletes will lie too because they want to play. Blood gives you a very objective measure of a number of different things.”

The tests conducted by these teams are voluntary.

The players’ union is currently in discussions with the NBA about blood testing, according to a source. 

“At the end of the day they’re here to help you and you’re signing it,” Tim Hardaway Jr. said, before the Knicks traded him to another team that uses Orreco. “If you feel like you’re putting yourself at risk then don’t do it. You’re the one that’s signing your name on it and your information is going to be out there at the end of the day if you do it. We know at the end of the day they’re not here to hurt us, they’re here to help us and help us try to get better each and every day as an athlete and help us be healthy.”