Synovial Fluid Aspiration

Indications:

Examination of synovial fluid is indicated in the investigation of chronic joint pain or distention. Fluid evaluation can be useful in the differentiation of inflammatory from degenerative arthropathies, and occasionally in the detection of neoplastic disease. Examination is also indicated in cases presenting with multiple joint involvement, suspected Lymes disease, immune mediated disease and in cases of shifting lameness, even where joint distention is not a clinical feature.

Restraint:

Requirements will vary dependent upon the extent of joint pain, the joint to be sampled and the tractability of the patient. In many cases, sedation +/- analgesia only is required.

Site Preparation:

Full surgical field preparation is recommended. However, some centres in the USA have reported successful sampling with minimal pre-operative preparation and no increased incidence of post-operative infection.

Equipment:

• 1" 20-22 gauge needle
• 2-5 ml syringe
• clean glass slides

Sample Collection:

The patient is positioned in lateral recumbency with the affected joint uppermost. Palpation of the joint during manual flexion and extension may help to identify the joint space. The needle should be advanced gently toward and through the joint capsule, avoiding damage to ligaments or the articular cartilage. Once the needle has been positioned, gentle negative pressure should be applied and the sample observed as it enters the syringe.

Haemorrhage associated with sampling is common and can be identified as the sample is collected as an uneven red cell presence or the sudden emergence of blood in an otherwise clear sample. Prior haemorrhage is associated with an even red or yellow colouration that does not vary during sample collection. It is important to record the gross appearance of the fluid during collection on the sample request form to aid differentiation of sampling associated from prior haemorrhage.

Sample volume will vary with the site of collection and the size of the patient. In small joints, prolonged sample collection is likely to be associated with significant haemorrhage, which may affect interpretation of the results. It is preferable to collect a smaller volume of relatively uncontaminated sample wherever possible.

Sample Handling:

Abnormal or blood-contaminated samples may clot after collection. Samples should be transferred into an EDTA container as soon as possible and the sample thoroughly mixed in the normal way.

In addition to the EDTA sample, it is helpful to prepare squash preparations of the fluid as described below. Slides should be placed in a plastic transporter (available from the laboratory upon request) and submitted together with the EDTA sample and submission form.

Smear-making:

The aim of smear-making is to display the cells in a monolayer while causing minimal disruption. Rapid drying of the smears is important in all preparations to maintain the cellular morphology, but this is particularly important in viscous fluids (such as synovial fluid). The use of a hairdryer (cool/warm, but not hot) will facilitate rapid drying. The air should be directed onto the back of the glass slide from a distance of at least 6 inches.

Squash Preparations:

This method is ideal for synovial fluid, tracheal and nasal washes and bone marrow aspirates. A small amount of the sample is placed on one slide and a second slide placed on top. The second slide may be perpendicular or parallel to the first. The material will spread between the two slides (Figure 1a). If necessary, very gentle pressure may be used to facilitate spreading (Figure 1b) and the top slide is gently pulled across the bottom until the two slides are separated (Figures 1c and 1d). The slides should slide apart, and should not be lifted away from each other.

Figure 1