Equine Synovial Fluid: Synoviocentesis and Analysis

Synoviocentesis
For accurate interpretation, a synovial fluid sample without iatrogenic haemorrhage or contamination is necessary. A number of techniques should be used to minimise these risks. The area should be clipped and aseptically prepared before aspiration. The patient should be adequately restrained (appropriate bit/sedation). Minimising patient movement is important and placement of a subcutaneous bleb of local anaesthetic at the proposed site of aspiration is recommended. The site selected for aspiration should be remote from any traumatic wounds/punctures and, when possible, areas of soft tissue swelling/infection. This reduces the risk of blood contamination and inoculation of the synovial cavity from periarticular sites of infection. Knowledge of sites for joint injection, including both dorsal and palmar/plantar pouches is essential. Use of a (relatively) large bore needle (19 gauge) minimises needle blockage by fibrin or congested synovium.

After needle insertion, if fluid flows freely this should be caught before any attempt at aspiration is made. If suction is required, this should be applied gently with a 5 ml syringe. Aspirated fluid should be collected into an EDTA blood collection tube for cytology and total protein determination and into an enrichment tube (biphasic blood culture medium) or lysing tube for culture and sensitivity. If a sample cannot be obtained (e.g. because of a draining wound), a wash sample (obtained following injection of sterile physiological fluid) may provide some information (i.e. white cell differential). Dilution of the synovial fluid can subsequently be estimated by determination of serum and sample urea (Gough et al. 2002), although in practice this is infrequently performed. [...]