Abnormal Pap Test Results: Understanding ASC-US, LSIL, HSIL, and the next steps

When women receive cervical cancer screening results (Pap tests) containing terms such as ASC-US, LSIL, or HSIL, they may feel anxious and uncertain about their health. However, understanding these terms is essential to gaining an accurate perspective and avoiding unnecessary worry. This article explains each type of result in detail and outlines the appropriate next steps.

1. What Is a Pap Test and Why Is It Important for Cervical Cancer Screening?

Cervical cancer screening

A Pap test (Papanicolaou test) is an effective screening method for detecting abnormal or precancerous cells in the cervix at an early stage. Regular screening has contributed to a significant reduction in cervical cancer incidence and mortality worldwide [1, 2]. Cervical cancer is preventable, and when detected early, it has a high cure rate, with a 5-year survival rate of up to 92% [2].

How is a Pap test performed?

A Pap test involves collecting cells from the cervix using a brush or sampling device. The cells are then spread onto a glass slide or placed in a preservative solution before being sent to a laboratory. There, cytology specialists examine the cells under a microscope to identify any abnormal changes [3].

The Bethesda System (TBS)

The Bethesda System (TBS) was developed to standardize the reporting of Pap test results. It classifies squamous cell changes associated with human papillomavirus (HPV) infection into categories such as ASC (atypical squamous cells) and SIL (squamous intraepithelial lesion) [4]. This system helps physicians better understand the severity of cellular abnormalities and determine appropriate management strategies [5].

Classification of cervical cytology according to the Bethesda System

2. Understanding Abnormal Pap Test Results

ASC-US (Atypical Squamous Cells of Undetermined Significance)

• Meaning: ASC-US is the most common abnormal Pap test result. It indicates the presence of atypical squamous cells, but the changes are insufficient to classify them as benign or as a low-grade (LSIL) or high-grade (HSIL) lesion [6].
• Possible causes: ASC-US may occur for several reasons, including infections (bacterial or fungal), inflammation, mucosal atrophy, or other benign reactions. HPV infection is one of the common causes of an ASC-US result [6].
• Prevalence and risk: Approximately 10–20% of women with ASC-US are found to have HSIL on biopsy [6]. The immediate risk of CIN3+ (high-grade precancerous lesions) in women with ASC-US who test positive for HPV is 4.5% [5].

2.1. LSIL (Low-Grade Squamous Intraepithelial Lesion)

• Meaning: LSIL indicates mildly abnormal squamous epithelial cells, usually associated with HPV infection. It corresponds to cervical intraepithelial neoplasia grade 1 (CIN 1) [4]. These changes generally have a favorable course and a high likelihood of spontaneous regression.
• Association with HPV: Most cases of LSIL are associated with high-risk HPV infection. However, most HPV infections, particularly in younger women, clear spontaneously within several months to a few years [7].
• Likelihood of spontaneous regression: Approximately 90% of LSIL lesions in young women may regress spontaneously within 2–3 years [8]. However, approximately 12–16% of women with LSIL are found to have CIN 2 or more severe lesions during colposcopy [9].

2.2. HSIL (High-Grade Squamous Intraepithelial Lesion)

• Meaning: HSIL indicates more severe cellular abnormalities corresponding to CIN 2 or CIN 3. These are considered precancerous lesions that carry a high risk of progressing to cervical cancer if left untreated [4, 10].
• Risk of progression to cancer: HSIL carries a substantially higher risk of progression to invasive cancer than LSIL. An estimated 30–50% of HSIL cases may progress to invasive cancer if left untreated [10].
• Role of HPV 16 and 18: HPV types 16 and 18 are among the highest-risk types and account for a large proportion of HSIL and cervical cancer cases [11].

3. What to Do After Receiving an Abnormal Pap Test Result

An abnormal result does not always mean cancer

It is important to remember that an abnormal Pap test result does not mean that a person has cervical cancer. In many cases, particularly with ASC-US and LSIL, abnormal cells may disappear spontaneously without treatment [1]. Even HSIL, although considered a precancerous lesion, can be treated effectively when detected early, preventing progression to invasive cancer [10].

The importance of follow-up and specialist consultation

After receiving an abnormal Pap test result, it is essential to follow the physician’s recommendations. The physician will develop a follow-up or treatment plan based on the result, the patient’s age, and other risk factors. This helps ensure that any cellular changes are managed promptly [5].

Additional tests

Depending on the Pap test result, the physician may recommend the following tests:

• HPV DNA testing: Detects the presence of high-risk HPV types. This test is particularly useful when the Pap test result is ASC-US or as a primary screening test for women over 30 years of age [12].
• Colposcopy: A procedure that uses a magnifying instrument to examine the surface of the cervix, vagina, and vulva in detail. The physician looks for abnormal areas and may take a biopsy if necessary [3].
• Biopsy: If colposcopy reveals an abnormal area, the physician will collect a small tissue sample and send it to the anatomical pathology laboratory. This is the definitive method for determining the severity of cellular changes and identifying whether a lesion is precancerous or invasive cancer [3].

4. Follow-up and Treatment

Recommendations for follow-up and treatment based on Pap test results and additional tests are regularly updated by reputable healthcare organizations, including the American Society for Colposcopy and Cervical Pathology (ASCCP) [5].

Follow-up for ASC-US

• ASC-US with a positive HPV result: The physician will generally recommend colposcopy.
• ASC-US with a negative HPV result: The Pap test and HPV test may be repeated after 3 years [5].

Follow-up for LSIL

• Women aged 25 years and older: Colposcopy is generally recommended.
• Younger women (21–24 years): A repeat Pap test after 12 months may be recommended because LSIL in this age group has a high rate of spontaneous regression [9]. If subsequent results remain abnormal or show more severe changes, colposcopy will be recommended [5].

Follow-up for HSIL

HSIL always requires more active evaluation and management.

• Colposcopy and biopsy: These are necessary to determine the severity of the lesion.
• Treatment to remove the lesion: If biopsy confirms CIN 2 or CIN 3, treatments such as cervical excision using loop electrosurgical excision procedure (LEEP) or cold-knife conization, or laser ablation, may be performed to remove the abnormal cells [5].

Recommendations from reputable healthcare organizations (ASCCP)

ASCCP guidelines emphasize risk-based management tailored to each individual, taking into account Pap test results, HPV test results, and the patient’s medical history [5]. This approach helps optimize follow-up and treatment while avoiding unnecessary interventions.

5. Prevention and Regular Screening

HPV vaccination

HPV vaccination is the most effective measure for preventing infection with high-risk HPV types that cause cervical cancer. Vaccination is recommended for both males and females at appropriate ages, preferably before sexual activity begins [13].

Regular Pap testing

Even after HPV vaccination, regular Pap testing remains important because the vaccine does not protect against every cancer-causing HPV type, and screening helps detect abnormal changes early. The recommended screening frequency will be determined by the physician based on age, medical history, and other risk factors [1].

Healthy lifestyle

Maintaining a healthy lifestyle, including a balanced diet, avoiding smoking, limiting alcohol consumption, and practicing safer sex, can also help protect cervical health and reduce the risk of sexually transmitted infections [14].

6. Frequently Asked Questions (FAQ)

• Is a Pap test painful? A Pap test is generally not painful, although it may cause mild discomfort or a brief sensation of pressure during sample collection. The procedure is quick and usually takes less than a few minutes.
• Do I need to avoid sexual intercourse after an abnormal Pap test result? Whether sexual intercourse should be avoided depends on the specific result and the physician’s recommendations. Complete abstinence is not necessary in many cases, but condom use may be advised to reduce the risk of HPV transmission or reinfection.
• What can I do to increase the likelihood of my Pap test result returning to normal? Many mild abnormalities, including ASC-US and LSIL, may regress spontaneously. Maintaining a healthy immune system through a balanced diet, regular exercise, and smoking cessation may support this process. Always follow the physician’s recommended follow-up schedule.
• Is cervical cancer screening necessary if I have received the HPV vaccine? Yes. The HPV vaccine does not protect against every HPV type that causes cervical cancer and does not treat existing HPV infections. Therefore, regular Pap testing remains important even for vaccinated individuals.
• When should I return for follow-up after an abnormal Pap test result? The physician will specify the follow-up schedule based on the type of abnormal Pap test result, HPV DNA test results (if available), age, and medical history. It is important to adhere to the scheduled appointments.

7. References

• [1] PDQ Screening and Prevention Editorial Board. (2025). Cervical Cancer Screening (PDQ®) - PDQ Cancer Information Summaries. NCBI Bookshelf.
• [2] Bengtsson, E., & Malm, P. (2014). Screening for Cervical Cancer Using Automated Analysis of PAP-Smears. PMC.
• [3] Widdice, L. E., & Moscicki, A.-B. (2008). Updated Guidelines for Papanicolaou Tests, Colposcopy, and Human Papillomavirus Testing in Adolescents. PMC.
• [4] Wang, T., Zhang, H., Liu, Y., & Zhao, C. (2023). Updates in Cervical Cancer Screening Guidelines, The Bethesda System for Reporting Cervical Cytology, and Clinical Management Recommendations. PMC.
• [5] Khieu, M., & Butler, S. L. (2023). High-Grade Squamous Intraepithelial Lesion of the Cervix. NCBI Bookshelf.
• [6] Wang, T., Zhang, H., Liu, Y., & Zhao, C. (2023). Updates in Cervical Cancer Screening Guidelines, The Bethesda System for Reporting Cervical Cytology, and Clinical Management Recommendations. PMC.
• [7] Widdice, L. E., & Moscicki, A.-B. (2008). Updated Guidelines for Papanicolaou Tests, Colposcopy, and Human Papillomavirus Testing in Adolescents. PMC.
• [8] Widdice, L. E., & Moscicki, A.-B. (2008). Updated Guidelines for Papanicolaou Tests, Colposcopy, and Human Papillomavirus Testing in Adolescents. PMC.
• [9] Widdice, L. E., & Moscicki, A.-B. (2008). Updated Guidelines for Papanicolaou Tests, Colposcopy, and Human Papillomavirus Testing in Adolescents. PMC.
• [10] Khieu, M., & Butler, S. L. (2023). High-Grade Squamous Intraepithelial Lesion of the Cervix. NCBI Bookshelf.
• [11] Khieu, M., & Butler, S. L. (2023). High-Grade Squamous Intraepithelial Lesion of the Cervix. NCBI Bookshelf.
• [12] PDQ Screening and Prevention Editorial Board. (2025). Cervical Cancer Screening (PDQ®) - PDQ Cancer Information Summaries. NCBI Bookshelf.
• [13] Wang, T., Zhang, H., Liu, Y., & Zhao, C. (2023). Updates in Cervical Cancer Screening Guidelines, The Bethesda System for Reporting Cervical Cytology, and Clinical Management Recommendations. PMC.
• [14] Widdice, L. E., & Moscicki, A.-B. (2008). Updated Guidelines for Papanicolaou Tests, Colposcopy, and Human Papillomavirus Testing in Adolescents. PMC.

The information above is for reference purposes only and is not intended as medical advice. Please contact your doctor for detailed medical consultation.