Most abnormal Pap tests are caused by human papillomavirus (HPV) infections. Other types of infection-such as those caused by bacteria, fungi (yeast), or protozoa (Trichomonas)-sometimes lead to minor changes on a Pap test called atypical squamous cells of undetermined significance (ASC-US). The most common cause is HPV infection. Natural cervical cell changes (atrophic vaginitis) related to menopause can also cause an abnormal Pap test.

• The cervix contains two kinds of cells: rectangular-shaped columnar cells on the surface of the cervix and in the cervical canal; and flat, scalelike squamous cells on the surface of the cervix.
• Columnar cells are constantly changing into squamous cells in an area of the cervix called the transformation zone.
• Because cells in this area of the cervix are always dividing, they are at risk for abnormal changes. Most abnormal cell changes found during a Pap test are from the transformation zone.