General pathology · The pathology laboratory
Cytology: the Pap test, fine-needle aspiration and body fluids !
- Technique
Cytology looks at loose cells that were scraped, aspirated or floating in a fluid; it is cheap and fast, ideal for screening.
Updated 2026-11-01
What it is
Cytology studies single cells, not tissue. They are obtained by scraping a surface (the Pap test), by fine-needle aspiration of a nodule, or from fluids such as pleural fluid, urine or cerebrospinal fluid. It is the basis of cervical cancer screening and of the work-up of a thyroid nodule, but it does not see architecture, so it cannot prove invasion.
Structure
- Koilocyte: a squamous cell with a clear halo around an enlarged, wrinkled, dark nucleus; the sign of HPV.
- Dysplastic cell: large, dark nucleus with an irregular outline and little cytoplasm.
- Papillary thyroid carcinoma on aspiration: nuclei with grooves, pseudoinclusions and pale chromatin.
- Cell block: the sediment of a fluid is embedded in paraffin so immunohistochemistry can be done.
Step by step
- Cells are obtained: cervical brushing, aspiration of a nodule or centrifugation of a fluid.
- They are smeared on a slide or suspended in a liquid medium that also allows HPV testing.
- They are stained; the Pap test uses the Papanicolaou stain.
- The cytologist assesses nuclear size and shape, the nuclear-to-cytoplasmic ratio and the chromatin.
- The result is reported in standardised categories, each linked to a specific next step.
- An abnormal result is confirmed on tissue: colposcopy and biopsy, or surgery.
Cytology categories to know
| Sample | Category | What it means |
|---|---|---|
| Cervix | ASC-US | Atypical squamous cells of undetermined significance; resolved by HPV testing or follow-up |
| Cervix | LSIL | Low-grade squamous intraepithelial lesion, often transient HPV infection (koilocytes) |
| Cervix | HSIL | High-grade squamous intraepithelial lesion; needs colposcopy and biopsy |
| Cervix | AGC | Atypical glandular cells; endocervix and endometrium must be studied |
| Thyroid | Benign | Colloid nodule or thyroiditis; follow-up |
| Thyroid | Follicular neoplasm | Adenoma cannot be separated from carcinoma: the surgical specimen is needed |
| Thyroid | Malignant | Usually papillary carcinoma, recognised by its nuclei |
| Fluids | Positive for malignancy | Tumour cells in the effusion: disseminated disease |
Clinical notes
- In Colombia and Latin America cervical cancer is still a major cause of cancer death in women; HPV testing is being added to cytology in screening.
- The Pap test screens: an abnormal result is not a cancer diagnosis but an indication for colposcopy.
- A thyroid nodule is studied with ultrasound-guided aspiration when its size or appearance calls for it.
- A malignant pleural effusion in a person with cancer changes the stage; cytology can be negative even with disease, so it is sometimes repeated.
Test yourself
Think of the answer, then tap the question to see it.
Which cell on a Pap test indicates an HPV effect?
The koilocyte: a squamous cell with a clear perinuclear halo and an enlarged, wrinkled, dark nucleus.
What follows a Pap test reported as HSIL?
Colposcopy with biopsy of abnormal areas to confirm the lesion and rule out invasion.
Why can cytology not diagnose an invasive carcinoma?
Because it studies loose cells without tissue architecture, and invasion is only seen in tissue.
Which thyroid carcinoma is diagnosed well by fine-needle aspiration?
Papillary carcinoma, by its nuclear features: grooves, pseudoinclusions and pale chromatin.
Related
- Biopsy: excisional, incisional, core needle, punch and fine-needle aspiration
- Dysplasia and carcinoma in situ
- Frozen section: the intraoperative consultation
- Immunohistochemistry: the principle and the markers to know
- Oncogenic viruses and H. pylori: the table to know
- Precancerous conditions: when chronic inflammation, metaplasia or hyperplasia raise the risk
Related conditions
- Cervical intraepithelial neoplasia (cervical dysplasia)
- Cervical cancer
- Human papillomavirus (HPV) infection
- Thyroid cancer (papillary)
- Thyroid nodule
- Follicular, oncocytic (Hürthle cell) and anaplastic thyroid carcinoma
- Bladder cancer
- Lung cancer
- Malignant pleural mesothelioma
- Breast cancer
- Cervical lymphadenopathy
- Endometrial cancer
Elsewhere in DurgiPedia
- Cervix and vagina: the transformation zone
- Thyroid follicles, C cells and parathyroid
- Simple, stratified, pseudostratified and transitional epithelium
Microorganisms
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Pap Smear: MedlinePlus Medical Test (MedlinePlus, US National Library of Medicine, 2024); Fine needle aspiration of the thyroid : MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2024); Cervical Cancer (StatPearls (NCBI Bookshelf), 2024); Thyroid Nodule (StatPearls (NCBI Bookshelf), 2024).
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