Fine-needle biopsy is used to sample diseased tissue for microscopic evaluation of cells (ie, cytology), which may be helpful in the initial or definitive diagnosis of infection, neoplasia, or other clinical states. Fine-needle biopsy may be performed with a needle attached to a syringe (ie, fine-needle aspiration) or with only a needle (ie, fine-needle capillary sampling). Fine-needle aspiration is preferred for sampling tissue with normal or low blood vascularity as well as tissue containing fibrous stroma. However, fine-needle capillary sampling may be used to reduce blood contamination when the lesion is suspected to be highly vascularized (eg, thyroid gland, hemangiosarcoma) or when the aspiration pressure results in ruptured cells (eg, cells of the thyroid gland and some lymphomas).
This discussion will focus on fine-needle biopsy of solid peripheral tissues, such as peripheral lymph nodes and solid skin lesions. Sampling internal tissues may require confirmation of adequate hemostasis (eg, adequate platelet concentration, normal coagulation testing), patient sedation, ultrasonographic guidance, and longer biopsy needles. Aspiration of fluid-filled lesions will require collection of the fluid into a tube containing EDTA (purple top) for optimal cytologic evaluation and/or into a tube without anticoagulant (red top) for culture or chemistry testing. Of note, the author recommends aspiration into EDTA only for aspiration of fluid (eg, cysts, vitreous samples) but not for samples suspected to be of low volume.
Some of the most common sample preparation pitfalls include:
- Applying too much vacuum pressure when performing fine-needle aspiration, thereby lysing cells
- Applying too much pressure when making compression preparations, thereby lysing cells
- Applying too little pressure when making compression preparations of thicker preparations
- Sampling nondiagnostic areas of the lesion, such as necrotic centers or superficial ulcers
Quick stains can be used to evaluate slides with an in-house microscope, but most clinical pathologists prefer evaluating cytology samples with the reference laboratory’s stain. If the samples are collected for evaluation by a pathologist, allow at least one slide to remain unstained.