Pap test results

Atypical squamous cells of undetermined significance (ASC-US)

This article was last reviewed by Adnan Karavelic, MD FRCPC on September 16, 2019

Quick facts:

  • ASC-US is a diagnosis that falls in between normal and the non-cancerous condition called low grade squamous intraepithelial lesion (LSIL).
  • Causes of ASC-US include human papillomavirus (HPV) infection, inflammation of the cervix, postmenopausal status, and prior radiation therapy.
  • A women diagnosed with ASC-US should be offered a repeat Pap test in 6 months to see if the condition has resolved.

The normal cervix

The cervix is part of the female genital tract. It is found at the bottom of the uterus where it forms an opening into the endometrial cavity. The narrow passage that runs through the cervix from the endometrium to the vagina is called the endocervical canal.

The part of the cervix inside the vagina is called the exocervix. It is covered by special cells called squamous cells. These cells form a barrier called the epithelium that protect the cervix. The endocervical canal is covered by different kinds of cells that connect to form endocervical glands. The area of the cervix where the exocervix meets the endocervical canal is called the transformation zone.

Why is this important? Most cancers of the cervix start in the transformation zone.

Why is the Pap test performed?

The Pap test is a screening test designed to look for pre-cancerous and cancerous cells in the cervix. Read our introduction article to learn more about the Pap test.

What does ASC-US mean?

ASC-US stands for atypical squamous cells of undetermined significance. ASC-US is a diagnosis that falls in between normal and the non-cancerous condition called low grade squamous intraepithelial lesion (LSIL). The diagnosis of ASC-US is made when your pathologist sees cells that share some but not all of the features of cells in LSIL.

Why is this important? The cells in ASC-US are not cancer cells.

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

For most patients, the condition that caused the changes seen in ASC-US will resolve on its own over time. However, because ASC-US can be associated with LSIL or other more serious conditions such as high-grade squamous intraepithelial lesion (HSIL) or squamous cell carcinoma, your doctor should arrange for you to have another Pap test in 6 months.

Why is this important? All women diagnosed with ASC-US should be offered a repeat Pap test in 6 months to see if the condition has resolved and to look for other more serious conditions.

How do pathologists make this diagnosis?

When examined under the microscope, the abnormal squamous cells in ASC-US have larger nuclei and slightly increased ratio of nuclear to cytoplasmic area compared to nearby normal squamous cells. The nuclei can also be slightly darker and have irregular nuclear membranes. Pathologists call cells that look darker than other cells hyperchromatic.

ASC-US in women under 30 years of age

According to the current guidelines, women who are < 30 years of age and diagnosed with ASC-US should have a repeat Pap test in 6 months. If ASC-US is diagnosed 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 and diagnosed 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 a 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 gynaecologist and family physician is required.

Colposcopy

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.

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