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
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.