The lab technology only looks for eggs to verify a parasite infection. Twice I have take samples of worms to the doctor and both came back negative, even though the stool sample was covered in white long worms. I made some investigation with doctors and they told me that noone actually looks at the sample. It goes straight into the lab machine which looks for and detects for EGGS. So if there are no eggs in the sample, there are "no parasites"