Bethesda system for cervical cytology

The Bethesda system is a way of naming what shows up on a Pap test, also called cervical cytology: a look at cells collected from the cervix, the lower part of the uterus. It isn't the test itself but the vocabulary used to write up the result, and it's the one used in the United States today. Each term in that vocabulary describes what kind of cells were found.

Those cells come from the transformation zone, the spot where the two linings of the cervix meet. Picture the seam where the patio floor meets the kitchen floor: if you sweep right along it, your dustpan picks up bits of both. That's why the sample holds two kinds of cells: squamous cells, which are flat like scales, and glandular cells, which make mucus. The system rates each kind separately, like a ladder that climbs from minor abnormalities to more serious ones.

When nothing looks out of the ordinary, the result reads NILM: negative for intraepithelial lesion or malignancy. An intraepithelial lesion is an area of abnormal growth within the lining, and malignancy means cancer. For squamous cells, ASC-US stands for atypical squamous cells of undetermined significance. Atypical means the cells don't look entirely normal, and undetermined significance means there are changes that suggest a lesion, but not enough to say for sure that one is there. LSIL and HSIL name actual squamous intraepithelial lesions, low-grade and high-grade. And for glandular cells, AGC stands for atypical glandular cells. Each of these is simply an abbreviation of its full English name.