Your doctor or health provider performs a Pap smear primarily to look for cervical and/or vaginal cells that are cancerous or could be potentially pre-cancerous. A sample of cervical cells or cell suspension is placed on a glass slide, stained with a special dye (Pap stain), and viewed under a microscope by a cytotechnologist or pathologist. The Pap test can also be used to detect vaginal or uterine infections. Abnormal cells and infections can be present without causing any noticeable symptoms. Some may require treatment while others may resolve on their own, prompt further testing, or be monitored over time. Some strains of human papillomavirus (HPV), a very common sexually transmitted viral infection, can be associated with an increased risk of cervical cancer. An HPV DNA test may be ordered along with or following a pap smear, especially for women over the age of 30.

Different organizations have varying opinions on the appropriate frequency of Pap testing (see Screening: Cervical Cancer (ages 13-18), (ages 19-29), (ages 30-49), (ages 50 and up). For example, the American Cancer Society recommends that annual screening with a Pap smear begin at the age of 21 or within three years of first becoming sexually active, whichever is sooner. In other situations, a Pap smear may be ordered if a woman has frequent sexual partners, is pregnant, or has abnormal vaginal bleeding, pain, sores, discharge, or itching. Screening is usually performed annually until a woman is 30 years of age. At this time, and after three consecutive normal Pap smears, it may be performed less frequently or as the doctor deems appropriate. The American College of Obstetricians and Gynecologists (ACOG) issued guidelines in August 2003 recommending that women 30 years of age or older be offered the option of having an HPV DNA test along with the Pap smear. If both are negative, testing may be performed less frequently.

A "negative" Pap smear means the cells obtained appear normal or there is no identifiable infection. In some instances, the conventional Pap smear may be reported as "unsatisfactory" for evaluation. This may mean that cell collection was inadequate or that cells could not be clearly identified. A summary of other reported results follows. [See also the sidebar on the 2001 Bethesda System for classification of Pap smear results.]

• Unsatisfactory: inadequate sampling or other interfering substance
• Benign: non-cancerous cells, but smear shows infection, irritation, or normal cell repair
• Atypical cells of uncertain significance: abnormal changes in the cells that cover most of the external part of the cervix (squamous cells-ASCUS) or in the cells that cover the lining of the uterus opening and canal (glandular cells—AGCUS) for which the cause is undetermined; an ASCUS test result is frequently followed up with DNA testing to identify the presence of a high-risk infection with HPV.
• Low-Grade changes: frequently due to infection with HPV, which in some instances can be a risk for cervical cancer; this test result may be followed up with DNA testing to identify the presence of a high-risk HPV infection.
• High-Grade changes: very atypical cells that may result in cancer
• Squamous cell carcinoma or adenocarcinoma: terms used to identify certain types of cancer; in these cases, cancer is evident and requires immediate attention.

According to the National Cancer Institute, about 55 million Pap smears are performed in the United States each year. Of these tests, about 3.5 million will be abnormal and require some degree of follow-up.

The Pap smear is generally used as a screening test. A certain percentage of abnormalities in women may go undetected with a single Pap smear, which is why it is important to have Pap smears done regularly. A significant limitation of the test has to do with sample collection. The Pap smear represents a very small sample of cells present on the cervix and in the vaginal area. Even for the most experienced physician, sample collection can be occasionally inadequate and a repeat Pap may be required.

The Pap smear, when performed routinely, has been a great help in the detection and treatment of areas of pre-cancer, which helps prevent cervical cancer from developing. In addition, the test helps detect cervical cancer in the early stages, when it is most treatable. The Pap smear is also used to monitor any abnormalities or unusual findings. In many cases, these findings are part of the body's repair process and often resolve themselves without any further treatment. If you douche, tub-bathe, or use vaginal creams 48 to 72 hours prior to the examination, your test results might be "unsatisfactory." Other factors that may alter results include menstrual bleeding, infection, drugs (such as digitalis and tetracycline), or having sexual relations within 24 hours prior to examination.

In these cases, a repeat Pap smear may be necessary, but it does not necessarily mean there is a significant problem. In some instances, the use of the liquid-based techniques may eliminate obscuring materials such as blood and mucus that may prevent a clear and uncluttered presentation of cervical cells. A second advantage is that the same sample may be used to perform additional testing for HPV, if appropriate.