Page 407 - WSAVA2018
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degradation of hyaluronic acid and thereby affect viscosity. Both EDTA and heparin are suitable for assessment of color and clarity and measurement of protein.
If a septic arthropathy is suspected synovial fluid should be submitted for aerobic culture in a blood culture bottle or if anaerobic culture is required then anaerobic transport media is needed.
Synovial fluid analysis
The clarity of synovial fluid is affected by the degree of cellularity. Normal synovial fluid is usually completely transparent. Synovial fluid may be graded as transparent, transluscent or opaque. Transparent synovial fluid allows the print on the syringe to be read through it. The print appears as an area of darkness if the fluid is transluscent and is indiscernible if the fluid is opaque.
Inflammatory fluid has a high white cell count and the fluid is usually opaque.
Viscosity may be assessed both subjectively and objectively. Viscosity is a function of the concentration and quality of hyaluronic acid and becomes poorer
as the degree of joint inflammation increases due to depolymerisation of the hyaluronic acid by bacterial or inflammatory proteases.
Subjective assessment of viscosity is performed at the time of fluid collection by observing the length of the synovial fluid strand created by dropping some fluid from the collecting syringe to a slide or alternatively by placing a drop of fluid between your thumb and forefinger. The viscosity is normal if a strand of greater than 2.5 cm
is achieved before breaking (although much longer strands are typical). Fluid of normal viscosity will not run off a microscope slide when held vertical.
Normal synovial fluid has viscosity like oil. A “thin” fluid of poor viscosity is common in inflamed joints, however it may also be seen in non-inflammatory arthropathies.
Objective assessment of viscosity by the mucin clot
test may be performed on synovial fluid collected into either plain or heparinised tubes although is not routinely performed. EDTA is unsuitable for the mucin clot test as it tends to degrade hyaluronic acid.
Total Cell Count
Total and differential cell counts of synovial fluid are important parameters of arthropathies. Reported total cell counts from normal canine joints can vary but are generally less than 3.0 x 109/L. Being a dialysate of plasma there are only small numbers of nucleated cells and an absence of red blood cells.
If there is only sufficient fluid to prepare a smear then a total cell count estimate should be. The body of a smear should contain one to three nucleated cells per 400x magnification field. Cell count estimates are graded as normal, or slightly, moderately or grossly increased.
Total white cell counts in non inflammatory arthropathies are typically normal to slightly elevated – usually < 5.0 x 109/L.
Total white cell counts in inflammatory arthropathies are typically moderately to markedly increased - > 10.0 x 109/L.
Synovial fluid is assessed grossly for volume, color, clarity and viscosity, and a total and differential white cell count and protein content performed. Mucin clot test is usually not indicated.
Normal values are listed below:
The volume of synovial fluid collected from a normal joint varies and ranges from 0. 1ml to 0.5ml. Joint effusion is usually apparent clinically +/- radiographically depending on the joint.
Normal synovial fluid should be colorless. A change in color suggests hemorrhage or inflammation. A yellow- tinge indicates that hemorrhage into the synovial membrane has occurred and hemoglobin breakdown products are being released into the fluid. Iatrogenic contamination with blood is apparent from incomplete mixing of blood as evidenced in a clear synovial fluid sample that becomes streaked and blood-tinged during collection. Should iatrogenic contamination occur this should be taken in to consideration when assessing total and differential white cell counts and protein levels as blood will falsely elevate all of these.
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