Information
Editors
Investigation of Synovial Fluid
Essentials
- 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.
- Ultrasound guidance is helpful; see video Knee Arthrocentesis and Glucocorticoid Injection.
- 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.
Investigation of synovial fluid
- Tests done with synovial fluid
- Cell counting: synovial fluid leukocytes + differential count from synovial fluid
- Microscopic examination to find any crystals
- Bacterial tests: bacterial culture and staining
- 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.