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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.

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