Information
Editors
Investigation of Synovial Fluid
Essentials
- Investigation of synovial fluid helps to establish the reason for joint swelling.
- The amount of synovial fluid is increased particularly in association with inflammation but also in patients with osteoarthritis or joint injuries.
- Clear, transparent synovial fluid suggests other than inflammatory joint disease, perhaps a structural defect (osteoarthritis, joint mouse, meniscus injury).
- In patients with arthritis, synovial fluid is more cloudy the more severe the infection (more leucocytes).
- If gout or bacterial arthritis is suspected, it is essential to investigate synovial fluid and this may give a specific diagnosis.
Synovial fluid
- Synovial fluid consists of plasma transudate with high molecular weight polysaccharide-containing molecules, particularly hyaluronates, produced by synoviocytes. In association with inflammation, leucocytes accumulate in synovial fluid.
Sampling
- It is usually easy to obtain a sample from a large joint producing synovial fluid but in smaller joints this may be difficult.
- Clean the sampling site with a disinfectant.
- It is advisable to use a larger-bore needle for sampling than for intra-articular injections.
- Ultrasound guidance will facilitate the puncture.
Sample containers
- Different laboratories may give different instructions regarding the stability and dispatch of samples.
Bacterial analysis
- For bacterial analysis, change the needle and inject the synovial fluid into an anaerobic transport vial (Portagerm® ) on top of the gel.
- Gram staining must be performed within 2 hours.
- If the sample is scanty, it should be taken to the laboratory immediately in the syringe.
- Culturing is of primary importance.
- If the sample volume is sufficient, in the laboratory some of it will be dropped directly from the syringe onto a slide under sterile conditions and used for gram staining and the rest transferred to a Portagerm® and used for culture.
Crystal analysis
- Collect the sample into a lithium heparin tube (crystals will remain stable for 3 hours).
- A small sample drop can be placed on a glass slide and covered with a cover slip, sealing the edges with colourless nail polish. This will keep the sample stable for a longer time.
Cell counting
- If the sample can be taken quickly to the laboratory, it should be collected into a lithium heparin tube.
- The problem here is the short period of stability of cells (1-2 hours).
- The same sample can also be used for crystal analysis.
- If it will take longer to get the sample to the laboratory, it should be collected into an EDTA tube (cells will remain stable for 6 hours).
- An EDTA sample cannot be used for crystal analysis.
Examination
- Examination of synovial fluid samples is divided into
- visual inspection
- cell counting
- microscopic examination to find any crystals
- bacterial examinations (gram staining, bacterial culture).
- For the purpose of treatment, examinations giving a specific diagnosis (bacterial or crystal arthritis) are of primary importance.
Inspection
- Note the colour, clarity and (less importantly) viscosity of synovial fluid aspirated into the syringe.
Colour
- The fluid is normally pale yellowish.
- Bleeding into the joint may stain the fluid red or orange.
- In a severely inflamed joint, due to the high number of leucocytes, synovial fluid may be light grey.
- Masses of urate crystal may make synovial fluid white.
Clarity
- Normal synovial fluid is clear.
- Leucocytes cause various degrees of cloudiness depending on the severity of the infection (number of leucocytes).
- Inspection of synovial fluid alone can be sufficient to differentiate between joint swelling due to osteoarthritis or, for instance, a meniscus defect (pale yellow, clear fluid) and swelling due to arthritis (cloudy fluid).
Viscosity
- Synovial fluid is normally viscous but in patients with inflammation enzymes break up polysaccharide-containing molecules, lowering the viscosity.
- When dropped from a syringe, normal synovial fluid will be drawn out to a 'thread', whereas inflammatory fluid will fall in drops.
Bacterial analysis
- Bacterial culture of synovial fluid should be performed if bacterial arthritis is suspected.
- As the culture results will only be obtained after 2 to 9 weekdays, the decision on starting antimicrobial medication must be made on clinical grounds.
- Emergency gram staining of synovial fluid (within 2 hours after drawing the sample) should also be done in such cases.
- In patients with bacterial arthritis, the causative agent can be found in synovial fluid in about 50% of cases.
Crystal analysis
- In patients with gout, urate crystals are needle-shaped and strongly negatively birefringent.
- A negative result will not exclude gout.
- In patients with pseudogout, calcium pyrophosphate crystals are rhomboid-shaped and weakly positively birefringent.
- After intra-articular glucocorticoid injections, strongly anisotropic glucocorticoid crystals may be found in synovial fluid for as long as 10 weeks (the laboratory should be informed of any such injections).
- Other crystals may be found in synovial fluid, such as lipid crystals, which are of no diagnostic significance.
Cell counting
- Normal synovial fluid contains less than 200 cells (× 106 /l).
- In patients with osteoarthritis or joint trauma, there are 200-10 000 cells (leucocytes), of which less than 50% are granulocytes (the fluid being clear or almost clear).
- In patients with inflammation, there are more than 2 000 leucocytes.
- Leucocyte levels exceeding 30 000 are normally seen only in patients with rheumatoid arthritis, gout, or reactive or bacterial arthritis. In such cases, the cells are mostly granulocytes.
- Cell levels exceeding 50 000 suggest bacterial arthritis but at an early stage levels can be lower.