Cytopathology

Cytopathology is a specialised branch of pathology concerned with the study and diagnosis of disease at the cellular level. It focuses on the microscopic examination of individual cells or small clusters of cells to detect abnormalities in structure, organisation, and behaviour. The discipline plays a central role in modern diagnostic medicine, particularly in the early detection of cancer, the evaluation of inflammatory and infectious diseases, and the assessment of reactive or degenerative cellular changes.
The term derives from Ancient Greek, with kytos meaning hollow or cell, pathos meaning disease or suffering, and logia meaning study. Cytopathology was formally established as a diagnostic field in 1928 by George Nicolas Papanicolaou, whose work led to the development of the Papanicolaou smear, commonly known as the Pap test.

Scope and Distinction from Histopathology

Cytopathology differs fundamentally from histopathology. While histopathology examines whole tissues with preserved architecture, cytopathology evaluates free cells or tissue fragments without the broader tissue context. This distinction allows cytopathology to be less invasive, faster, and often more cost-effective, though sometimes less precise in architectural assessment.
Cytopathology is occasionally and inaccurately referred to as cell biology. However, cell biology is a broad scientific discipline studying cellular structure and function in health and disease, whereas cytopathology is a clinical diagnostic specialty focused on disease identification.

Clinical Applications

Cytopathology is widely used to investigate diseases affecting numerous body sites. Its most prominent application is in cancer diagnosis and screening, but it is also valuable in identifying infectious, inflammatory, autoimmune, and degenerative conditions.
A classic example is the Pap smear, a screening tool designed to detect cervical intraepithelial neoplasia and early cervical cancer. Cytopathology also aids in diagnosing malignancies of the thyroid, breast, lung, lymph nodes, urinary tract, and body cavity fluids.
Cytopathologic tests are often referred to as smear tests, as samples are commonly spread across a glass microscope slide and stained for examination. However, modern cytology employs a variety of preparation techniques beyond simple smearing.

Methods of Cell Collection

Cell collection for cytopathologic analysis is broadly divided into exfoliative cytology and interventional cytology.

Exfoliative Cytology

Exfoliative cytology involves the collection of cells that have been shed from epithelial surfaces, either spontaneously or through mechanical means.
Spontaneous exfoliation occurs when cells naturally slough off into body fluids, such as:

• Pleural fluid
• Peritoneal fluid
• Pericardial fluid

These fluids can be collected and examined for malignant, inflammatory, or reactive cells.
Mechanical exfoliation involves the deliberate removal of cells from a surface. Common examples include:

• Pap smears, where cells are scraped from the cervix using a spatula or brush
• Bronchial brushings, obtained during bronchoscopy to sample airway lesions

These techniques are particularly useful for screening and early detection of epithelial abnormalities.

Interventional Cytology

Interventional cytology involves active sampling of cells from lesions or masses by penetrating the body.
The most widely used technique is fine-needle aspiration cytology (FNAC). FNAC uses a thin hypodermic needle, typically 23 to 27 gauge, attached to a syringe to aspirate cells from a lesion. Negative pressure may be applied to increase cellular yield.
FNAC can be performed:

• Under palpation guidance for superficial lesions such as those in the neck, thyroid, or breast
• With imaging guidance, such as ultrasound or computed tomography, for deep-seated lesions

FNAC is minimally invasive, causes little tissue damage, and often provides rapid diagnostic information. Diagnostic accuracy is highest when the procedure is performed or immediately assessed by a pathologist, allowing inadequate samples to be re-aspirated promptly.

Additional Cytologic Techniques

Other specialised cytologic collection methods include:

• Sediment cytology, where cells shed into fixative during biopsy or autopsy processing are centrifuged and examined
• Imprint cytology, where fresh tissue is pressed onto a glass slide, leaving an imprint of cells for rapid evaluation

These techniques are particularly useful for intraoperative consultation and post-procedural assessment.

Specimen Preparation and Staining

After collection, cytology specimens are processed to preserve cellular detail and enhance visual contrast. Two principal preparation approaches are used:

• Direct smearing onto glass slides
• Suspension of cells in liquid media for liquid-based cytology

Improper preparation may result in artefacts that obscure diagnostic features. To visualise cellular components clearly, specimens are stained using established cytological stains.
Common staining methods include:

• Papanicolaou stain, ideal for nuclear detail and widely used in gynaecological cytology
• Romanowsky-type stains, such as Giemsa, Wright, Jenner, Field, May–Grünwald, and Leishman stains

Each stain highlights different cellular structures, aiding in comprehensive evaluation.

Cytological Parameters and Diagnostic Criteria

The cell nucleus is the most critical structure in cytopathologic assessment. Malignant transformation is often associated with altered DNA activity, which manifests as visible nuclear changes.
Key nuclear features assessed include:

• Increased nuclear size
• Hyperchromasia, with darker-staining nuclei
• Irregular nuclear contours
• Coarse or uneven chromatin distribution
• Prominent or multiple nucleoli

In addition to malignancy, cytopathologists may identify:

• Microbial infections, including bacterial, viral, and parasitic organisms
• Reactive and inflammatory changes
• Immune-mediated cellular responses
• Degenerative and ageing-related cellular alterations

Role in Clinical Decision-Making

Cytopathology is most effective when integrated with clinical examination and medical imaging. Together, these three modalities provide a comprehensive diagnostic framework.
One major advantage of cytopathology is its ability to spare patients from unnecessary surgery. For example, many benign thyroid nodules can be confidently diagnosed with FNAC, allowing patients to avoid invasive procedures and resume normal activities promptly.
When malignancy is detected, cytology can guide further management, including surgery, radiotherapy, chemotherapy, or combined approaches. However, certain tumours, such as sarcomas, may be difficult to diagnose cytologically due to low cellular yield or architectural dependence. In rare cases, biopsy may be hazardous, as with pheochromocytoma.

Advanced Diagnostic Techniques

Modern cytopathology increasingly incorporates ancillary testing. When specimens are prepared appropriately, additional investigations may include:

• Immunocytochemistry
• Molecular diagnostic testing
• Flow cytometry, particularly in lymphoid neoplasms

Reflex testing is common, such as human papillomavirus testing following abnormal cervical cytology.

Body Regions Commonly Examined

Cytopathologic techniques are applied across virtually all organ systems, including:

• The female reproductive tract
• The urinary tract, including ureters, bladder, and urethra
• Serous cavities such as the pleura, peritoneum, and pericardium
• The breast
• The thyroid gland
• Lymph nodes
• The lungs and airways
• The alimentary tract