A diagnosis of Atypical Squamous Cells of Undetermined Significance (ASCUS) on a Pap test can be concerning, but it is important to understand what this result truly signifies. ASCUS is a common finding in cervical screening that indicates a minor cellular abnormality requiring follow-up. It does not diagnose cancer, but it flags the need for additional monitoring to determine the cause of the cell changes and ensure a more serious condition is not present.
Defining Atypical Squamous Cells of Undetermined Significance
Atypical Squamous Cells of Undetermined Significance is a term used to describe cells collected from the surface of the cervix during a Pap test that appear slightly abnormal under a microscope. The cells in question are squamous cells, which are the flat, thin cells lining the outer part of the cervix. The word “atypical” means that these cells do not look entirely typical or normal, showing variations in size or shape.
The critical phrase in the diagnosis is “undetermined significance,” which indicates the cytologist cannot definitively determine the cause of the changes. The cellular alterations are not severe enough to be classified as a low-grade or high-grade precancerous lesion. This abnormality could be due to various factors, including a minor infection, inflammation, or hormonal changes.
ASCUS is the most frequently reported abnormal result from a Pap test, occurring in approximately 2 to 5% of all screenings. The finding may represent a benign, temporary condition, or it could be the first sign of a low-grade precancerous condition. For this reason, ASCUS requires a specific follow-up plan to clarify the nature of these cellular changes.
The Connection Between ASCUS and HPV
Human Papillomavirus (HPV) infection is the primary underlying cause of most ASCUS findings in cervical screening. HPV is a common virus, and certain types are known to cause cellular changes in the cervix that can lead to an ASCUS reading. The virus causes these cells to look slightly irregular, which is what the pathologist observes during the Pap test.
HPV strains are divided into high-risk (oncogenic) and low-risk types. High-risk HPV types, such as HPV-16 and HPV-18, are responsible for nearly all cases of cervical cancer and are of greater concern when found alongside an ASCUS result.
HPV testing is the standard next step to stratify the risk, as it clarifies whether the cellular changes are linked to a high-risk viral infection. An ASCUS result that is positive for high-risk HPV carries a higher potential for underlying precancerous lesions than one that is negative.
Next Steps After an ASCUS Result
The management of an ASCUS result is focused on risk stratification, which is primarily determined by the presence or absence of high-risk HPV. For women aged 25 and older, the preferred strategy is “reflex HPV testing,” where the same sample used for the Pap test is analyzed for high-risk HPV strains.
Negative HPV Result
If the high-risk HPV test is negative, the risk of developing a serious precancerous lesion is very low. Guidelines recommend a return to routine screening, typically with a repeat Pap test in three years. This longer interval is considered safe because the negative HPV result provides strong reassurance.
Positive HPV Result
If the high-risk HPV test is positive, the risk of having or developing a precancerous lesion is higher, necessitating a more immediate follow-up procedure. The standard next step is a colposcopy. During a colposcopy, the physician uses a specialized magnifying instrument to closely examine the cervix and take targeted biopsies if abnormal areas are identified.
For adolescents and young women between the ages of 21 and 24, management is often more conservative, typically involving a repeat Pap test in 12 months, as HPV infections frequently clear spontaneously in this age group.
Understanding the Likelihood of Progression
Despite the need for follow-up, an ASCUS result generally carries a very favorable long-term prognosis. The majority of ASCUS findings, particularly those not associated with high-risk HPV, will resolve spontaneously without medical intervention. Studies show that for women with ASCUS, the risk of having a high-grade lesion, defined as CIN 2 or worse, is approximately 9.7%.
When managed according to guidelines, the progression to invasive cervical cancer is extremely rare. This low likelihood is due to the slow nature of cervical carcinogenesis, which typically takes five to ten years to progress from precancerous changes to invasive cancer.
The management strategy of using HPV testing to triage ASCUS results is highly effective at identifying the small subset of women who need treatment. By adhering to the recommended surveillance schedule, any developing precancerous lesion can be detected and treated long before it has the opportunity to progress to invasive disease.