Increased amounts of synovial fluid occur particularly in association with inflammation but also in patients with osteoarthritis or joint injuries.
Aspiration of synovial fluid from a swollen joint immediately gives valuable information about the cause of the joint problem.
Clear, transparent synovial fluid suggests something 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 the synovial fluid is cloudy, a sample should be taken for laboratory tests.
If gout or bacterial arthritis is suspected, it is essential to investigate synovial fluid and this may give a specific diagnosis.
N.B.! If there is clinical suspicion of purulent arthritis (such as in monoarthritis with fever), emergency referral to a hospital for synovial fluid sampling and bacterial tests is indicated.
Synovial fluid
Synovial fluid consists of plasma filtrate containing high-molecular-weight polysaccharide-containing molecules, particularly hyaluronates, produced by synoviocytes.
Mechanical irritation and inflammation of the joint increase the amount of synovial fluid, and inflammation, in particular, increases the number of leukocytes present.
Sampling
Prepare all the equipment required for taking a synovial fluid sample, and ask for an assistant, as necessary.
Depending on the joint affected, the patient can either sit or lie down with the joint supported.
Clean the sampling site with an antiseptic.
Wear sterile protective gloves for puncturing large joints, at least.
Local anaesthesia of the puncture site is usually not necessary. If the patient is fearful, you can apply an anaesthetic ointment or anaesthetize the skin using a thin needle.
If you inject an anaesthetic, the joint cavity should be punctured at a different site
It is advisable to use a larger-bore needle for sampling than for intra-articular injections. After emptying the joint, the needle can be left in place and used to inject a glucocorticoid into the joint.
If gout is suspected, the first metatarsophalangeal joint can be punctured with a blue needle (23G).
For larger joints, black (22G; wrist, ankle) or yellow (20G; shoulder, knee) needles can be used.
Pink needles (18 G) are suitable for aspirating large volumes of fluid.
After withdrawing the needle, press the site lightly with a sterile dressing to avoid the formation of a haematoma from tissue damage caused by the puncture. Cover the puncture site with a dressing, and ask the patient to keep it clean and dry until the evening.
Visual inspection of synovial fluid
Note the colour, clarity and - less importantly - the viscosity of the synovial fluid aspirated into the syringe. Even visual inspection alone will help with differential diagnosis.
Colour
The fluid is normally pale yellowish.
Bleeding into the joint may stain the fluid red or orange. Artefact blood associated with sampling will show as red streaks.
In a severely inflamed joint, due to the high number of leukocytes, synovial fluid may be light grey, in purulent arthritis greenish or brownish orange.
Masses of urate crystal may make synovial fluid white.
Clarity
Normal synovial fluid is transparent.
Leukocytes cause various degrees of cloudiness depending on the severity of the infection (number of leukocytes).
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).
Very cloudy synovial fluid suggests purulent arthritis or gout. If so, bacterial tests and crystal analysis of synovial fluid are indicated.
Viscosity
Synovial fluid is normally viscous but in patients with inflammation enzymes break up polysaccharide-containing molecules, lowering the viscosity.
When dripped from a syringe, normal synovial fluid will be drawn out to a 'thread', whereas in rheumatic inflammation, the fluid will fall in drops.
Sample handling and sample containers
Different laboratories may give slightly different instructions regarding sample containers and the stability and dispatch of samples.
If the synovial fluid sample can be taken quickly to the laboratory, it should be collected into a lithium heparin tube (green plastic cap).
The problem here is the short period of stability of leukocytes but, on the other hand, in this case the same sample can also be used for crystal analysis.
If it will take longer to get the synovial fluid sample to the laboratory, it should be collected into an EDTA tube where cells will remain stable for about 6 hours.
An EDTA sample cannot be used for crystal analysis.
For 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, attaching it to the slide with colourless nail polish. This will keep the sample stable for a longer time.
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 applied directly from the syringe under sterile conditions for gram staining and the rest transferred to a Portagerm® and used for culture.
For bacterial nucleic acid detection, the synovial fluid sample should be collected into an empty tube.
As necessary: synovial bacterial nucleic acid detection, Borrelia burgdorferi nucleic acid detection, Mycobacterium tuberculosis staining, culture and nucleic acid detection
For the purpose of treatment, examinations giving a specific diagnosis (bacterial or crystal arthritis) are of primary importance.
Bacterial analysis
Bacterial culture and staining
Gram staining of synovial fluid must be performed within 2 hours.
As the culture results will only be obtained after 2 to 9 weekdays, the decision on starting antimicrobial medication must be based on gram staining and clinical assessment.
Nucleic acid detection (bacteria, borrelia) can be done with a sample collected into an empty tube.
Crystal analysis
Crystals in synovial fluid
In patients with gout, sodium 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.
As strongly anisotropic glucocorticoid crystals may be found in synovial fluid for as long as 10 weeks after intra-articular glucocorticoid injections, the laboratory should be informed of any such injections.
Also other crystals may be found in synovial fluid, such as lipid crystals, which are of no diagnostic significance.
Cell counting
Synovial fluid leukocytes and differential count
Normal synovial fluid contains less than 200 cells (× 106 /l).
In patients with osteoarthritis or joint trauma, there are 200-10 000 cells (leukocytes), of which less than 50% are granulocytes (the fluid being clear or almost clear).
In patients with inflammation, there are more than 2 000 leukocytes.
Leukocyte 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.