Fine needle aspiration cytology (FNAC)

Principle

• A thin needle collects cells from a mass or organ; smears are stained and examined for diagnosis.
• It samples cells rather than intact tissue, so it assesses cellular features but only limited architecture.

Specimen

• Palpable lesions (thyroid, breast, lymph node, salivary gland, soft tissue) or deep lesions under ultrasound, CT or endoscopic ultrasound guidance.

Reagents and equipment

• Fine needles (typically 22–25 gauge), 10–20 mL syringes with or without a syringe holder, slides, 95% alcohol or spray fixative.
• Rapid Romanowsky stain (e.g. Diff-Quik), Pap and H&E stains; saline or RPMI for needle rinses; formalin for cell blocks.

Procedure

• Confirm patient identity and consent; clean the skin.
• Insert the needle into the lesion and move it back and forth in several directions, with suction (aspiration) or without suction (capillary technique).
• Release suction before withdrawing the needle; make 2–4 passes as needed.
• Expel material onto slides and spread gently to make thin smears.
• Air-dry some smears for Romanowsky stains (cytoplasm, background) and wet-fix others immediately in 95% alcohol for Pap stain (nuclear detail).
• Rinse the needle for cell block, flow cytometry, culture or molecular tests.
• Rapid on-site evaluation (ROSE) checks adequacy before the patient leaves.

Quality control and pitfalls

• Blood dilution, thick or crushed smears and delayed fixation (air-drying artifact in Pap smears) reduce quality.
• Sampling error can miss a lesion; a non-diagnostic or benign result does not exclude malignancy when clinical suspicion is high.
• Some diagnoses need histology, e.g. follicular carcinoma of thyroid requires capsular or vascular invasion.
• Use standardized reporting systems (e.g. Bethesda for thyroid, Milan for salivary gland).

Clinical use

• Rapid, inexpensive, minimally invasive diagnosis of lumps, lymphadenopathy and deep masses, with material for ancillary tests.
• Complications are uncommon: hematoma, pain, infection and pneumothorax for chest lesions.