What are koilocytes?


Koilocytes are squamous cells that look abnormal under the microscope. They are most commonly found in tissues infected with the human papillomavirus (HPV). These cells have a distinctive appearance that helps pathologists recognize HPV-related changes.

What do koilocytes look like under the microscope?

When examined under the microscope, koilocytes show several characteristic features:

  • Enlarged nucleus – The central part of the cell (nucleus) is bigger than normal.

  • Irregular nuclear shape – The edges of the nucleus may look uneven or wrinkled.

  • Perinuclear clearing – There is a clear space or “halo” around the nucleus.

  • Thickened cell edge – The outer border of the cell may appear more prominent.

These features together create the classic look of a koilocyte.

Where are koilocytes found?

Koilocytes are usually found in squamous epithelium, which is the tissue that lines surfaces such as the cervix, vagina, vulva, penis, anus, and throat. Pathologists most commonly describe koilocytes in Pap tests (cervical screening samples) or in biopsies taken from mucosal surfaces.

Why are koilocytes important?

The presence of koilocytes is strong evidence of HPV infection. Since some types of HPV are linked to the development of precancerous conditions and cancers (such as cervical cancer), finding koilocytes in a sample may lead to further testing or closer follow-up.

However, koilocytes alone do not mean cancer. They indicate that HPV has caused visible changes in the squamous cells, and doctors will consider other findings such as the degree of dysplasia (abnormal growth) when deciding what it means for your health.

What diseases are associated with koilocytes?

Koilocytes are seen in several conditions caused by HPV infection. These include:

  • Condyloma acuminatum (genital warts) – A benign growth on the genital or anal area caused by low-risk types of HPV.

  • HPV-associated squamous papillomas – Benign growths that can occur in areas such as the mouth, throat, or genital region.

  • Recurrent respiratory papillomatosis – A rare condition where warty growths form in the airways, also linked to low-risk HPV types.

  • Low grade squamous intraepithelial lesion (LSIL) – A precancerous condition in the cervix or other mucosal sites strongly associated with the presence of koilocytes.

  • High grade squamous intraepithelial lesion (HSIL) – A more advanced precancerous condition. Koilocytes may still be seen, but they are less common as the cells become more abnormal.

In each of these conditions, the presence of koilocytes helps pathologists confirm HPV-related changes.

Questions to ask your doctor

  • Were koilocytes found in my sample?

  • Does the presence of koilocytes mean I have HPV?

  • Do I also have dysplasia or precancerous changes?

  • What follow-up or additional testing do you recommend?

  • Does this result change how often I should have screening tests?

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