Why would you want to fool around with a blood test?   You have to draw the blood, and, I'm not sure if you send the sample in or do it yourself, but you still have wait.   If she's open you have to back through and find her.   Just too much hassle.

If you palpate them, you reach in, you know in less than a minute, and you move the girl to correct pen and she's done.   Move on to the next one.