The Pap smear is a routine screening procedure designed to detect abnormal cell changes on the cervix, the lower part of the uterus that opens into the vagina. This test is a highly effective tool in preventive medicine, allowing for the early identification of potentially precancerous or cancerous conditions long before they develop into invasive disease. By collecting a sample of cells and examining them under a microscope, the Pap smear provides a snapshot of cervical health, which is important because most cases of cervical cancer are preventable with timely detection.
Understanding the Bethesda System Framework
The results of a Pap smear are communicated using the standardized Bethesda System (TBS), which was established to ensure consistent and clear communication between laboratories and healthcare providers. This framework classifies findings into three overarching result categories, moving from normal health toward more significant cellular changes. The first and most desired category is “Negative for Intraepithelial Lesion or Malignancy” (NILM), which indicates no cellular evidence of a precancerous lesion or cancer was found.
The second category, “Epithelial Cell Abnormalities,” is the core of an abnormal Pap smear result and is further subdivided to indicate the specific type and severity of abnormal cells. This grouping includes the five specific precancerous findings that are the main focus of follow-up care. The third category, “Other Malignant Neoplasms,” is reserved for rare instances where invasive cancer is suspected or identified.
The Mildest Abnormalities Atypical Squamous Cells
The first two specific categories of abnormal epithelial cells fall under the term “Atypical Squamous Cells” (ASC), which are the most frequently encountered abnormal results. These cells appear slightly irregular under the microscope, but the changes are not distinct enough to definitively classify them as a precancerous lesion. The “atypical” designation signifies that while the cells look mildly unusual, this appearance can be caused by various factors, including inflammation, hormonal changes, or a minor infection.
Atypical Squamous Cells of Undetermined Significance (ASC-US)
This is the mildest specific abnormality and the most common abnormal Pap smear finding. This result often suggests a low-risk human papillomavirus (HPV) infection, which is the underlying cause for most cervical cell changes, or irritation that is likely to resolve on its own, especially in younger individuals. Management for ASC-US often involves an HPV test to determine if a high-risk strain of the virus is present or a repeat Pap test in a few months to see if the cells have returned to normal.
Atypical Squamous Cells—Cannot Exclude High-Grade Squamous Intraepithelial Lesion (ASC-H)
This less common finding warrants closer investigation. This category is used when the abnormal cells are more concerning than ASC-US but still lack the clarity for a definitive diagnosis of a high-grade lesion. The name reflects the uncertainty of the finding while acknowledging a higher risk. Due to the elevated potential for a more severe underlying condition, an ASC-H result typically prompts an immediate recommendation for a colposcopy.
Low-Grade and High-Grade Precancerous Lesions
Moving past the atypical cell findings, the next categories represent true precancerous changes, which are classified based on the severity of the cellular abnormality.
Low-Grade Squamous Intraepithelial Lesion (LSIL)
The third specific category is Low-Grade Squamous Intraepithelial Lesion (LSIL), which indicates mild changes, or mild dysplasia, in the cells lining the outer part of the cervix. LSIL is highly associated with an active HPV infection and is generally considered a low-risk lesion. The cell changes are confined to the surface layer, and in many cases, this finding regresses spontaneously as the immune system clears the viral infection.
High-Grade Squamous Intraepithelial Lesion (HSIL)
The fourth category, High-Grade Squamous Intraepithelial Lesion (HSIL), indicates a more advanced and concerning change, encompassing moderate to severe dysplasia. HSIL cells look much more abnormal and disorganized than LSIL cells, and the changes affect deeper layers of the cervical lining. This finding carries a substantially higher risk of progressing to invasive cervical cancer if left untreated. Although HSIL is not cancer itself, it represents a significant precancerous condition that requires prompt diagnostic follow-up and often treatment to remove the abnormal tissue.
Atypical Glandular Cells (AGC)
The fifth category covers Atypical Glandular Cells (AGC), which are distinct from the squamous cells, as they originate from the mucus-producing cells in the endocervical canal or the lining of the uterus. AGC is a relatively rare finding on a Pap test, but it carries a higher potential risk of underlying high-grade lesions or cancer than atypical squamous cells. Because these cells originate higher up, the evaluation must be more comprehensive, sometimes requiring examination of the uterine lining as well as the cervix. AGC is associated with both adenocarcinoma in situ (a glandular precancer) and invasive adenocarcinoma.
Next Steps After an Abnormal Result
Receiving an abnormal Pap smear result initiates diagnostic procedures tailored to the specific finding, moving from least to most invasive based on the indicated risk. For the mildest result, ASC-US, the next step is often reflex HPV testing or a repeat Pap test in 6 to 12 months, relying on the high chance of the abnormality resolving spontaneously. If the result is a low-grade lesion (LSIL), follow-up is often a colposcopy or a repeat Pap and HPV co-test, depending on the patient’s age and previous results.
A colposcopy is the standard procedure for more serious findings like ASC-H, HSIL, and all AGC results. This in-office procedure uses a specialized magnifying instrument to examine the cervix. The clinician applies a solution that highlights areas of abnormal cell growth. If suspicious areas are identified during the colposcopy, a small tissue sample (biopsy) is taken for definitive diagnosis.