Atypical squamous cells of undetermined significance (ASC-US) of the cervix

by Adnan Karavelic MD FRCPC
March 13, 2022

What does atypical squamous cells of undetermined significance (ASC-US) mean?

Atypical squamous cells of undetermined significance (ASC-US) means that abnormal-looking squamous cells were seen on your Pap test. ASC-US is a preliminary result and not a final diagnosis. Conditions associated with ASC-US include non-cancerous changes such as inflammation in the cervix and the pre-cancerous disease low grade squamous intraepithelial lesion (LSIL). The cells in ASC-US are not cancer cells.

What causes ASC-US?

ASC-US is a relatively common Pap test result in women of all ages. Causes of ASC-US include infection with human papillomavirus (HPV), inflammation of the cervix, postmenopausal status, and prior radiation therapy.

What do the cells in ASC-US look like under the microscope?

When examined under the microscope, the abnormal squamous cells in ASC-US have larger nuclei and the cytoplasm (body of the cell) is smaller relative to the nucleus compared to the healthy squamous cells normally found in the cervix.  Pathologists describe this as an increased nuclear to cytoplasmic ratio. The nuclei can also be slightly darker and have irregular nuclear membranes. Pathologists call cells that look darker than other cells hyperchromatic.


What happens after a diagnosis of ASC-US?

ASC-US in women under 30 years of age

According to the current guidelines, women who are < 30 years of age with ASC-US should have a repeat Pap test in 6 months. If ASC-US is seen again, another Pap test is performed in 6 months. If ASC-US still persists, your doctor should refer you to a specialist who will perform a colposcopy.

ASC-US in women 30 years of age or older

For women 30 years of age or older with ASC-US, the tissue collected during the Pap test should be sent for human papillomavirus (HPV) testing. This test looks for specific high-risk types of HPV that are known to cause cervical cancer.

If a high-risk type of HPV is found in your sample, your doctor should refer you to a specialist who will perform a colposcopy. Finding high-risk HPV does not mean that you have cervical cancer. It does mean, however, that you are at higher risk of developing a pre-cancerous or cancerous growth, and that a closer follow-up with your gynecologist and family physician is required.


A colposcopy allows your doctor to see the entire outer surface of the cervix. During the colposcopy, the doctor will be looking for any areas that look abnormal on the surface of the cervix. If an abnormality is found, the doctor may decide to take a small sample of tissue called a biopsy to look for pre-cancerous and cancerous changes. Your doctor may also take a biopsy from the endocervical canal and endometrium.

Other helpful resources:

Choosing Wisely Canada

Cancer Care Ontario

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