Fine needle aspiration cytology (FNAC)

What it is

Fine needle aspiration cytology (FNAC) is a minimally invasive test that uses a 22 to 25 gauge needle to remove cells from a lump, lymph node, cyst, or organ for microscopic examination. Its main advantage is that it is usually quick, low-cost, and can often be done in an outpatient setting, but the key limitation is that it collects cells rather than intact tissue architecture. That means FNAC can strongly suggest benign disease, infection, inflammation, recurrence, or cancer, yet in some situations it cannot fully classify a tumour or replace a core needle biopsy or surgical biopsy.

In practice, FNAC is commonly used for palpable breast lumps, enlarged lymph nodes, thyroid nodules, salivary gland lesions, and suspected metastatic deposits. The sample is spread on glass slides, stained, and examined by a cytopathologist. In many centres, ultrasound guidance is used when the target is small, deep, or difficult to feel. In India, FNAC is widely used because it is relatively accessible and can help speed referral to surgery, oncology, or other specialist care.

A simple comparison is:

Procedure	What is taken	Main strength	Main limitation
FNAC	Individual cells	Fast, minimally invasive, inexpensive	No tissue architecture
Core needle biopsy	Small tissue cores	Better tumour typing and receptor testing	More invasive than FNAC
Surgical biopsy	Larger tissue sample	Most complete histology	Most invasive

How it works

A clinician inserts a thin needle into the abnormal area and applies suction, or sometimes uses a capillary technique without suction, to collect cells. The material is then expelled onto slides or into preservative for cytology preparation. Depending on the site, the procedure may be done by a pathologist, radiologist, surgeon, or other trained clinician.

Ultrasound guidance improves targeting for many non-palpable or partly cystic lesions. For example, in breast and lymph node assessment, imaging can help the needle sample the most suspicious part of a lesion and avoid necrotic or fluid-only areas that may give a non-diagnostic result.

The pathologist looks at cell size, shape, arrangement, nuclear features, background material, and sometimes uses ancillary tests. In selected cases, the aspirated material can also be used for immunocytochemistry, flow cytometry, microbiology, or limited molecular testing. Even so, the amount of material is smaller than with a core biopsy, so not every downstream test is possible.

Evidence and uses

FNAC is an established diagnostic procedure in cancer care when used for the right clinical question. Reviews describe it as quick, accurate, and cost-effective in experienced hands, especially for confirming recurrence or metastasis, evaluating enlarged lymph nodes, and assessing some breast, thyroid, and salivary gland lesions.

Common uses include:

1. Lymph nodes: to assess enlarged nodes in the neck, armpit, groin, or elsewhere for reactive change, infection, lymphoma suspicion, or metastatic cancer.
2. Breast lesions: often as part of a triple assessment with clinical examination and imaging. It can rapidly identify cyst contents, benign changes, or malignant cells, but core biopsy is often preferred when architecture or receptor testing is needed.
3. Thyroid nodules: to help sort nodules into benign, suspicious, or malignant categories. FNAC is particularly good at detecting papillary thyroid carcinoma, but it cannot reliably distinguish follicular adenoma from follicular carcinoma because that distinction depends on capsular or vascular invasion seen on histology.
4. Suspected recurrence or metastasis: for example, a new lump in a patient with prior cancer.
5. Salivary gland and superficial soft-tissue lesions: where a quick preliminary diagnosis may guide further treatment.

Diagnostic performance varies by organ, operator skill, sample adequacy, and pathology expertise. A strong result can be very helpful, but a non-diagnostic or indeterminate result does not rule out cancer. False negatives can happen if the needle misses the lesion, samples a necrotic area, or the lesion sheds few diagnostic cells. False positives are less common but can occur because some benign and malignant conditions look similar on cytology.

For this reason, FNAC works best when interpreted alongside the clinical examination and imaging findings rather than in isolation. If the cytology result does not fit the scan or the doctor's examination, further testing is usually needed.

Diagnosis / how it's measured

FNAC itself is a diagnostic sampling method, not a blood test. The result is usually reported in categories such as:

Possible report category	What it generally means
Non-diagnostic / inadequate	Not enough representative cells to interpret
Benign	No malignant cells seen; follow-up depends on context
Atypical / indeterminate	Abnormal cells present, but not clearly benign or malignant
Suspicious for malignancy	Strong concern for cancer, but not fully definitive
Malignant	Cancer cells identified

Adequacy matters. If too few cells are obtained, the report may be inconclusive and the procedure may need to be repeated, often with ultrasound guidance. Some centres use rapid on-site evaluation, where a trained professional checks sample adequacy immediately so extra passes can be taken during the same visit.

FNAC does not usually provide all the information needed for cancer staging or full tumour profiling. For many cancers, treatment planning also needs histopathology, immunohistochemistry, receptor status, or molecular markers, which are more reliably obtained from core biopsy or surgical specimens.

When to see a clinician

A clinician may suggest FNAC if you have a new lump, persistent enlarged lymph node, thyroid nodule, breast lesion, or a suspected recurrence after cancer treatment. You should also seek medical review if a lump is growing, hard, fixed, painful, associated with weight loss or fever, or persists beyond a few weeks.

Before the procedure, tell the team about blood thinners such as warfarin, apixaban, rivaroxaban, clopidogrel, or aspirin, as well as any bleeding disorder. Many FNAC procedures can still be done safely, but the team may need to adjust timing or apply extra precautions. Also mention prior imaging, biopsy results, tuberculosis history, or previous cancer, because these details change how the sample is interpreted.

After FNAC, mild soreness or bruising is common. Contact the treating team if you develop increasing swelling, significant bleeding, fever, redness, severe pain, or breathing difficulty after a neck procedure.

Limitations and open questions

The main limitation of FNAC is that it examines cells without preserving tissue architecture. This is why it may not distinguish some benign from malignant follicular-pattern thyroid lesions, may be less definitive for lymphoma subtyping, and may be insufficient for full biomarker testing in many solid tumours.

Accuracy depends heavily on technique, lesion type, imaging guidance, slide preparation, and cytopathology expertise. Published studies show wide variation in diagnostic accuracy and complication rates across settings. Evidence supports FNAC as a valuable first-line or problem-solving test, but not as a universal replacement for core biopsy.

Complications are usually minor, such as pain, bruising, or a small haematoma. Serious complications are uncommon but depend on the site sampled. Evidence in humans supports its safety overall, yet the exact risk profile differs for superficial versus deep organs.

An important open question in modern cancer care is how best to combine FNAC with newer ancillary tests. In some centres, cell-block techniques and molecular assays increase the usefulness of FNAC, but availability is uneven. Where pathology infrastructure is limited, FNAC remains especially useful because it can provide a rapid answer at relatively low cost, but clinicians still need to recognise when a more definitive tissue biopsy is necessary.