First available at Multigenre Fan Convention Pyrkon. It was part of promo set called "Krewni Pyrkonu", which was given as a reward for blood donation. Then as part of the Dice Tower 2020 Kickstarter Campaign.
The Blood Test deck will allow your characters to get another way of checking their Contamination cards. As you probably know by now, positive results are not always nice for your life expectancy.