At the beginning of the episode the men in hazmat suits are drawing blood from the dead bodies. They draw the blood from the arm and you can see the blood going into the vials. This is not the method for collecting blood samples from a corpse and would not provide an adequate sample to be tested. This is because the heart is no longer pumping blood through the circulatory system and there would not be enough blood in the arm to draw that amount of blood from a single vein. For this reason blood samples are not taken from a corpse until the time of an autopsy, and the samples are taken directly from the heart.