Fine needle aspiration techniques

Fine needle aspirates (FNA) are inexpensive and can be collected quickly and easily with little or no risk to the patient. Ultrasound guided aspirates allow sampling of focal

lesions and thoracic and abdominal organs or masses may be sampled with less risk of complications than for conventional core biopsy techniques. When using ultrasound guidance it is important that ultrasound gel (which may obscure all cellular detail in an aspirate) is removed prior to needle insertion.

Possible complications include haemorrhage, peritonitis, bacteraemia, fistula formation and tumour seeding but the risk is reported to be low for abdominal aspirates and in most cases is outweighed by the diagnostic benefit. However, evaluation for a bleeding disorder is recommended, particularly if a vascular structure is to be aspirated and there are some circumstances in which FNA is contraindicated (see below).

Cytological specimens collected by FNA lack the cellular architecture necessary to characterise many lesions as completely as by full tissue biopsy. FNA frequently helps to determine the type of pathologic process (inflammatory, neoplastic etc) but biopsy may be necessary to fully characterise a neoplastic process and therefore to determine the prognosis.

Lymph nodes

Fine needle aspirates of enlarged peripheral lymph nodes can usually be obtained without anaesthesia or sedation. A 23-25g 1-1½” needle and 5ml syringe can be used, or a needle alone. The needle-only technique may reduce haemodilution of the sample and lysis of fragile cells caused by aspiration.

First, swab the skin over the lymph node with alcohol. The lymph node is fixed between thumb and forefinger and the needle introduced. When using a syringe, negative pressure is applied whilst moving the needle through the lymph node to obtain a representative sample. Negative pressure is released before withdrawing the needle. Disconnect the needle and syringe, draw air into the syringe, re-attach, and carefully expel the sample onto two clean glass slides. Smears are then prepared using a blood smear technique (Fig 1), or gentle squash technique (Fig 2). Lymphoma cells are fragile and the blood smear technique often results in less cellular lysis than the alternative.

For the needle-only technique, a 25g needle is inserted into the lymph node and redirected/ retracted several times. Cells are collected by capillary action. The needle is withdrawn, a syringe containing air connected and the sample gently expelled onto slides. Smears are prepared as described above.

Cytological examination is most likely to be diagnostic in cases of lymphoma or metastatic neoplasia in which there is extensive involvement of the node. The submandibular lymph node is often reactive and should be avoided if there are other nodes which might be sampled. Reactive hyperplasia, lymphadenitis and many cases of lymphoma can be diagnosed with confidence on good quality aspirates but where there is doubt as to the diagnosis, then histopathology will be recommended to establish a diagnosis.

Fig 1 Blood smear technique ©NationWide Laboratories 2017

Fig 2 Squash preparation technique ©NationWide Laboratories 2017

Metastatic mast cell tumour ©NationWide Laboratories 2017

Lymph node aspirate ©NationWide Laboratories 2017

Liver

Indications

Fine needle aspiration of the liver has limited diagnostic utility and is predominantly used in the investigation of diffuse hepatomegaly or focal lesions. It may be useful for diagnosis of lymphoma and for identification of cytoplasmic change consistent with hepatic lipidosis and vacuolar hepatopathy. However, the technique is less useful for diagnosis of inflammatory hepatopathies and differentiation of benign and malignant hepatocellular or bile duct neoplasms.

The correlation between cytologic examination of fine-needle liver aspirates and histological examination of hepatic biopsy samples is reported to be poor. The diagnostic correlation of samples was 30% in dogs and 51% in cats in one study.

Normal hepatocytes ©NationWide Laboratories 2017

For diffuse hepatomegaly, a representative sample can be obtained without visual guidance (if ultrasound is not available) but focal lesions require visualisation (ultrasonography, laparoscopy). If a laparotomy is performed then a biopsy for histology is usually more appropriate. If larger tissue biopsies are taken, then impression smears of the biopsy surface may be made prior to fixation and allow for more rapid examination.

Canine Adenovirus intra-nuclear inclusion bodies ©NationWide Laboratories 2017

Contraindications

FNA is contra-indicated if there is any evidence of defective haemostasis. Although FNA cytology causes minimal tissue damage, platelets are most important in occluding defects in vessels. The platelet count should be >10 per field using the x100 oil objective, assessed in the monolayer of the smear. Coagulation studies are recommended prior to FNA or other biopsy techniques although some texts state that a coagulation screen is unnecessary if there was no haemorrhage associated with venipuncture. Aspiration techniques are also contraindicated where haemangiosarcoma is suspected. FNA of such lesions is unlikely to provide a diagnosis and risks rupture of the capsule.

Equipment

A needle with a stylet is preferable (for example a 2½” spinal needle with a stylet) as the stylet prevents obstruction of the needle by tissue from the body wall. The stylet is removed when the liver has been penetrated. A 1½” 22g hypodermic needle is adequate if a spinal needle is not available
A 2-5ml syringe
Clean glass slides

Blind aspiration technique

Ultrasound guided sampling is preferred but if this is not possible a blind aspiration technique may be used. Fine needle aspiration of the liver can be performed without sedation, depending on the temperament of the patient. Two alternative approaches are described:

Standing

Restrain the patient in a standing position
Prepare aseptically the 11th intercostal space on the RHS
Only 1-2ml of negative pressure should be applied Dorsal recumbency
Position the patient in dorsal recumbency with the body tilted slightly to the right and the head elevated (causes the liver to move caudally within the abdominal cavity)
Prepare aseptically a region on the left side including the xiphoid process medially and the costal arch laterally
Insert the needle midway between the left costal arch and the end of the xiphoid process at an angle of approximately 30-45 degrees to the skin. In cats the angle of insertion must be almost vertical to avoid penetrating the diaphragm
Advance the needle and after the liver has been penetrated apply about 5ml negative pressure as the needle is advanced through the liver. If blood appears in the syringe release the negative pressure, if aspiration is continued blood can dilute the sample and may clot, making it difficult to prepare smears
Gently release the suction before removing the needle from the abdominal cavity
Prepare smears by the routine blood smear technique (Fig 1) and rapidly air dry. If possible prepare several smears. Do not fix or cover-slip
If the sample contains too few cells, repeat

Intrahepatic biliary stasis. Bile plugs and lakes in canaliculi ©NationWide Laboratories 2017

Lung

Trans-thoracic fine needle aspiration may be considered if less invasive procedures, for example trans-tracheal or bronchial wash have not, or are unlikely to provide diagnostic material. Pulmonary aspirates can be obtained quickly and the likelihood of obtaining a useful or diagnostic sample is high particularly if accurate needle placement can be determined. Diffuse lesions often yield high quality samples while in generalised lung disease, multiple attempts may be required due to the aspiration of normal cellular components or non-representative areas. A definitive diagnosis may not be achieved but characterisation of inflammation versus neoplasia is often possible.

In very sick patients, FNA of the lung may be used to avoid higher-risk procedures such as thoracotomy or biopsy.