The surface of a woman's cervix is made up of cells known as squamous cells. In some cases, some cells may become abnormal; these are known as atypical squamous cells, or ASC. These abnormal cells may be caused by a variety of issues, some of more concern than others.

Atypical squamous cells are typically discovered during a routine Pap smear. This is a gynecological test where a doctor collects and examines cells from the cervix to determine if they are healthy. If any cells appear abnormal, this is referred to as dysplasia, which may be a precursor to cancer. In these cases, the doctor will perform further tests to find the root cause of the issue.

There are two categories of abnormal squamous cells; the first is known as ASC-US, or atypical squamous cells with uncertain significance. This means there are some minor changes in a portion of the cells, but the cause is not immediately clear. ASC-US is generally not considered to be dysplasia, as the abnormalities in the cells are very minimal. They may be the result of several relatively minor issues, including a yeast infection, hormonal changes, or the use of certain medications.

Pap results that fall under the ASC-US category typically are not an immediate cause for concern. A test for the human papillomavirus, or HPV, may be useful to determine if this is the cause, as it is often responsible for abnormal cells; it may also later contribute to cancer, so early diagnosis is desirable. If the woman is negative for HPV, a second Pap smear may be performed after several months to see if the atypical squamous cells are still present. Once a woman has had two normal Pap smears, she can return to regular routine exams. If the second Pap is again ASC-US, further tests will probably be required.

The second type of atypical squamous cell is known as ASC-H. The H means that the results suggest the possibility of a high-grade squamous intraepithelial lesion. These types of cells are extremely abnormal and may be precancerous, so further testing should be performed right away. The doctor will typically order a colposcopy, where the cells of the cervix are coated with a solution and then examined using an instrument called a colposcope. A biopsy may also be necessary to remove a sample of tissue for laboratory examination.