Joint and Bursa Aspiration

Aspiration is performed for diagnosis, for treatment, or for both at once. The most important diagnostic indication is a hot, swollen, or unexplained joint effusion, where the fluid is needed to exclude infection and to look for crystals. It is also used for a suspected haemarthrosis, for example after injury, and to sample a bursa. As a treatment, aspirating a tense, painful effusion or an enlarged bursa relieves pressure and pain, and it is often combined with an injection once infection has been excluded. In a hot, swollen joint, septic arthritis must be urgently considered and excluded, and aspiration is the key step in that assessment, while other urgent causes remain possible.

The technique follows the same safe principles as any injection, with some specific points. An aseptic non-touch technique is used, and the needle is never passed through overlying broken, inflamed or infected skin, to avoid seeding infection into the joint. A wider-bore needle rather than a fine one is used because effusions and especially a haemarthrosis can be too thick to draw through a fine needle, with the exact gauge guided by the site, the fluid viscosity and local protocol, and ultrasound guidance is helpful when the joint is difficult to access, deep, or has only a small effusion. A crucial exception is a prosthetic (replaced) joint: this is not aspirated in the general clinic, but referred urgently to orthopaedics and aspirated through the specialist pathway in an appropriate sterile environment, which may be image-guided, because of the serious consequences of introducing infection into a prosthesis. The olecranon and prepatellar bursae are common sites for bursa aspiration.

When a joint is aspirated for diagnosis, the synovial fluid (SF) is sent promptly to the laboratory for a defined set of tests. The essential investigations are a cell count and differential, which includes the white cell count (WCC); a Gram stain to look for bacteria; and culture and sensitivities to identify any organism and guide antibiotics; with examination for crystals under polarised light microscopy where crystal arthritis is possible. Samples are collected following local laboratory requirements, including inoculating blood culture bottles where advised. Where it can be done promptly, the fluid is sent before antibiotics are started, because antibiotics can render the culture negative, but antibiotics are not delayed in a septic or clinically unstable patient.

The appearance of the fluid gives an immediate clue. Normal synovial fluid is clear, pale straw-coloured and viscous, with a low white cell count. In inflammatory conditions it becomes cloudy and turbid with a higher white cell count, in infection it can be frankly purulent, and after bleeding into the joint it is bloodstained. A turbid or purulent appearance raises concern but cannot by itself diagnose infection, which is why the fluid is always sent for laboratory analysis. The crystal findings identify the crystal arthropathies: monosodium urate crystals, seen in gout, are long and needle-shaped and are negatively birefringent under polarised light, while calcium pyrophosphate crystals, seen in pseudogout (calcium pyrophosphate deposition), are shorter and rhomboid and are weakly positively birefringent. Reporting the fluid appearance, the white cell count and differential, the Gram stain and culture result, and the crystal findings together builds the diagnostic picture.

Interpreting synovial fluid results safely means understanding what the tests can and cannot tell you. A higher white cell count with a high proportion of neutrophils raises the probability of septic arthritis, but the counts seen in infection, crystal arthritis and other inflammatory conditions overlap, so no single count confirms or excludes infection. Two rules are particularly important and frequently tested. First, the presence of crystals does not exclude infection: gout or pseudogout and septic arthritis can coexist in the same joint, so finding crystals does not mean the joint is not also infected. Second, a negative Gram stain does not exclude septic arthritis, because the Gram stain misses a large proportion of culture-positive infections, so a negative result never rules infection out.

The practical consequence is that septic arthritis is a clinical diagnosis supported by, not decided by, the fluid results. If the clinical picture suggests infection, it is treated as an emergency regardless of the Gram stain: this means urgent senior and orthopaedic or rheumatology input, joint washout where indicated, and antibiotics started after cultures are taken where this does not delay treatment in a septic or unstable patient, because delay risks rapid, permanent joint damage. Where infection is excluded, the same fluid analysis distinguishes gout, pseudogout, an inflammatory arthritis and a non-inflammatory effusion such as osteoarthritis or an effusion after injury, and aspiration also gives symptomatic relief. Recognising a possible septic joint early and acting on it, rather than being falsely reassured by crystals or a negative Gram stain, is the key safety message.

• •Aspiration (arthrocentesis) is both diagnostic (analysing the fluid) and therapeutic (relieving a tense effusion or bursa); the key reason to aspirate a hot joint is to exclude septic arthritis.
• •Indications include a hot or unexplained effusion, suspected septic or crystal arthritis, and a haemarthrosis; in a hot, swollen joint, septic arthritis must be urgently considered and excluded, though other urgent causes remain possible.
• •Use an aseptic technique, do not pass the needle through infected or broken skin, use a wider-bore needle for thick fluid or blood, and use ultrasound if the joint is difficult; a prosthetic joint is referred urgently to orthopaedics for aspiration in an appropriate sterile environment, not tapped in the general clinic.
• •Send synovial fluid for cell count and differential (white cell count), Gram stain, and culture and sensitivities, with crystal examination where crystal arthritis is possible, following local laboratory requirements; send it before antibiotics where this does not delay urgent treatment.
• •Urate crystals (gout) are needle-shaped and negatively birefringent; calcium pyrophosphate crystals (pseudogout) are rhomboid and positively birefringent.
• •Crystals do not exclude infection and a negative Gram stain does not exclude septic arthritis, so a suspected septic joint is treated as an emergency with urgent referral, washout where indicated, and antibiotics after cultures.