# Fine Needle Aspiration: Technique and On-Site Adequacy

## Overview

Fine needle aspiration (FNA) is a minimally invasive diagnostic procedure that uses a thin needle (22-27 gauge) to obtain cellular material from palpable or image-guided lesions. The pathologist plays a critical role in performing or assisting with FNA, providing rapid on-site evaluation (ROSE), triaging material for ancillary studies, and rendering cytologic diagnoses. When performed with proper technique and on-site evaluation, FNA provides rapid, cost-effective diagnosis with minimal patient morbidity.

## FNA Technique

### Needle and Syringe Selection

The standard setup uses a 25-gauge needle attached to a 10-20 mL syringe held in a Cameco-type syringe holder. The non-aspiration (capillary) technique uses the needle alone without suction, relying on capillary action to draw cells into the needle lumen. This approach is preferred for vascular lesions, particularly thyroid nodules, where it reduces blood contamination that can obscure cellular detail. Larger needles (22-gauge) may be used for deep-seated lesions under image guidance. Core needle biopsy with 18-20 gauge needles is increasingly used in conjunction with or as a replacement for FNA, particularly when architectural information is needed.

### Procedure Steps

For palpation-guided FNA, the mass is stabilized between two fingers of the non-dominant hand while the needle is inserted and moved rapidly back and forth through the lesion in multiple directions to sample different areas. Negative pressure is released before withdrawing the needle to prevent aspiration of material into the syringe barrel where it cannot be recovered. Typically 3 to 6 passes are performed, with the number guided by on-site adequacy assessment. For image-guided procedures (ultrasound, CT, or endoscopic ultrasound), the radiologist or interventionalist performs the needle placement while the pathologist provides ROSE.

### Smear Preparation

Direct smears are the foundation of cytologic diagnosis. Material is expelled from the needle onto glass slides, and two types of preparations are made. Air-dried smears are stained with Romanowsky stains (Diff-Quik) and can be evaluated within seconds for ROSE; they excel at highlighting extracellular material such as mucin, colloid, and amyloid. Alcohol-fixed smears provide better nuclear detail and are optimal for Papanicolaou staining. Liquid-based preparations (ThinPrep or SurePath) are made by rinsing residual needle material into fixative solution, producing thin monolayer preparations useful for ancillary studies. Cell blocks are prepared from clotted or centrifuged material, processed as formalin-fixed paraffin-embedded tissue, and are critical because they allow immunohistochemistry and molecular testing.

### Triage of Material

Proper triage at the time of the procedure is essential and cannot be corrected retrospectively. The first pass typically goes to direct smears for ROSE assessment. Subsequent passes are allocated to cell block (critical for immunohistochemistry panels), flow cytometry (for lymphoma workup, requiring fresh tissue in saline or RPMI), microbiology (cultures for AFB, fungi), and molecular studies as clinically indicated.

## Rapid On-Site Evaluation (ROSE)

### Purpose

ROSE serves four main purposes: assessing specimen adequacy in real time, providing a preliminary diagnosis to guide clinical management, triaging material for appropriate ancillary studies, and reducing the rate of non-diagnostic or inadequate specimens from approximately 20% without ROSE to less than 5% with it.

### ROSE Technique

Diff-Quik stained air-dried smears are evaluated under the microscope within 1 to 2 minutes of preparation. The pathologist communicates directly with the proceduralist regarding whether the specimen is adequate or inadequate, provides a preliminary diagnostic impression, and indicates whether additional passes are needed for ancillary testing. ROSE findings are documented for quality assurance purposes.

### Adequacy Assessment

Adequacy criteria are site-specific. For thyroid FNA, the Bethesda criteria require a minimum of 6 groups of 10 or more follicular cells on at least 2 slides. For lymph node aspirates, sufficiency means adequate lymphoid cells have been obtained along with a flow cytometry specimen. For pancreatic FNA, diagnostic epithelial or stromal cells consistent with the clinical and imaging context constitute adequacy. A specimen is classified as inadequate or non-diagnostic when there is insufficient cellular material, obscuring blood, drying artifact, or poor preservation. Specimens are termed atypical or indeterminate when cellularity is adequate but findings are insufficient for a definitive diagnosis.

### Common Pitfalls in ROSE

Over-reliance on ROSE for final diagnosis is a common error; ROSE is preliminary only, and definitive diagnosis awaits review of all preparations. Reactive atypia in previously irradiated tissue can be misinterpreted as malignancy. Failure to triage material for flow cytometry when aspirating a lymph node means a potential lymphoma workup is irretrievably lost. Air-drying artifact on slides intended for alcohol fixation obscures nuclear detail and can render preparations uninterpretable.

## Site-Specific Considerations

### Thyroid FNA

The thyroid is the most common FNA site. Procedures are guided by ultrasound risk stratification (TI-RADS) and reported using the Bethesda System for Reporting Thyroid Cytopathology. The capillary technique is preferred to minimize blood contamination. For cystic lesions, the fluid should be aspirated completely and submitted for cytology, with subsequent re-aspiration of any solid component.

### Lymph Node FNA

The critical requirement in lymph node FNA is obtaining material for flow cytometry (submitted in saline or RPMI) to enable complete lymphoma workup. Touch preparations of core biopsy can supplement FNA material. When granulomas are identified on ROSE, material should be triaged for AFB and fungal cultures. When metastatic carcinoma is suspected, a cell block is prioritized for an immunohistochemistry panel to determine the primary site.

### Pancreatic FNA (EUS-guided)

Endoscopic ultrasound-guided FNA of pancreatic lesions is performed by the gastroenterologist with the pathologist providing ROSE. This partnership significantly reduces the non-diagnostic rate from approximately 30% to less than 10%. For solid lesions, ductal adenocarcinoma appears as three-dimensional clusters and single atypical cells, with desmoplastic stroma visible on cell block. For cystic lesions, fluid analysis (CEA and amylase levels) combined with cytology helps distinguish mucinous cysts from serous cystadenomas and pseudocysts. Neuroendocrine tumors present as monotonous plasmacytoid cells requiring triage for chromogranin and synaptophysin immunohistochemistry.

### Salivary Gland FNA

The Milan System for Reporting Salivary Gland Cytopathology provides standardized diagnostic categories for salivary gland FNA. Pleomorphic adenoma shows characteristic fibrillary myxoid stroma with embedded epithelial and myoepithelial cells. Warthin tumor demonstrates oncocytic cells with a lymphoid background and dirty proteinaceous debris. A persistent challenge in salivary gland cytology is distinguishing benign from low-grade malignant neoplasms, which may require excision for definitive classification.

### Lung FNA / Bronchial Washings

Lung lesions may be sampled by transthoracic CT-guided FNA or EBUS-guided FNA and biopsy. The cell block is critical for immunohistochemistry (TTF-1, p40, napsin A for tumor classification) and molecular testing (EGFR, ALK, PD-L1). Adequate material must be triaged for molecular testing, as the FNA or small biopsy is increasingly the sole diagnostic specimen available for treatment decisions.

<image>A medical illustration demonstrating the step-by-step technique of fine needle aspiration with smear preparation. Panel A: Cross-section showing a 25-gauge needle inserted into a palpable mass with the non-aspiration (capillary) technique, with an arrow indicating the back-and-forth sampling motion. Panel B: The material being expelled from the needle onto a glass slide. Panel C: A second slide placed over the drop and gently pulled apart to create a thin smear. Panel D: Two resulting preparations -- one air-dried slide being stained with Diff-Quik (blue-purple) for ROSE and one immediately placed in 95% ethanol for Papanicolaou staining. Panel E: The remaining needle rinse being placed into CytoLyt solution for liquid-based preparation and cell block.</image>

<image>A medical illustration showing rapid on-site evaluation (ROSE) findings from four different organ sites. Panel A: Thyroid -- clusters of follicular cells with colloid in the background (Diff-Quik stain), annotated as adequate/benign. Panel B: Lymph node -- monomorphic small lymphoid cells in a dispersed single-cell pattern with flow cytometry specimen being triaged into RPMI tube. Panel C: Pancreas (EUS-guided) -- three-dimensional cluster of ductal epithelial cells with nuclear crowding and loss of honeycomb architecture, annotated as suspicious for adenocarcinoma. Panel D: Salivary gland -- fibrillary magenta-colored myxoid stroma with embedded epithelial and myoepithelial cells, annotated as consistent with pleomorphic adenoma.</image>

## Clinical Pearls

The non-aspiration (capillary) technique produces less bloody specimens for thyroid FNA and is preferred by most experienced practitioners. ROSE reduces non-diagnostic rates and improves specimen triage, with studies demonstrating cost-effectiveness despite requiring dedicated pathologist time at the procedure. Always obtain material for flow cytometry when aspirating a lymph node, because even if the aspirate appears morphologically reactive, flow cytometry provides definitive assessment for lymphoproliferative disorders. Cell block is essential for immunohistochemistry and molecular testing, and cell block material should never be sacrificed for additional smears when ancillary testing is anticipated. For pancreatic cystic lesions, fluid CEA greater than 192 ng/mL has 75% sensitivity and 84% specificity for mucinous cysts, while cytology alone has only approximately 50% sensitivity. FNA has inherent limitations for diagnoses requiring architectural assessment, including follicular thyroid carcinoma, lymphoma subtyping, and soft tissue tumors, where core biopsy may be necessary. Communication between pathologist and proceduralist during ROSE is bidirectional: the clinician provides clinical context while the pathologist provides real-time guidance on the number of passes needed and the requirement for ancillary studies.

## References
- Cibas ES, Ducatman BS. *Cytology: Diagnostic Principles and Clinical Correlates*. 5th ed. Elsevier; 2021.
- Layfield LJ, et al. Immediate on-site interpretation of fine-needle aspiration smears: A cost and compensation analysis. *Cancer Cytopathol*. 2001;93(1):36-39.
- Schmidt RL, et al. Rapid on-site evaluation reduces inadequacy rates in thyroid fine-needle aspiration: A systematic review and meta-analysis. *Thyroid*. 2013;23(4):428-435.
- Pitman MB, et al. *The Milan System for Reporting Salivary Gland Cytopathology*. Springer; 2018.
- Klapman JB, et al. Clinical impact of on-site cytopathology interpretation on endoscopic ultrasound-guided fine needle aspiration. *Am J Gastroenterol*. 2003;98(6):1289-1294.