- careful exam to identify crystals in synovial fluid can establish a dx early and avoid unnecessary hospital admissions for treatment of suspected infectious arthritis.
- presence of crystals does not exclude infection, esp since abnormalities such as Gout may increase likelihood of septic arthritis;
- cultures should be performed on samples of synovial fluid if there is any concern about infection, even if crystals are present.
- Fat Droplets:
- large fat droplets in synovial fluid suggest frx involving marrow space;
- small lipid droplets, which occur in pancreatic-fat necrosis or fractures, can be misread by Coulter counters as leukocytes;
- liquid lipid crystals have characteristic Maltese-cross configuration
- look for brightly birefringent needles on compensated polarized microscopy;
- monosodium urate crystals may be seen by normal light microscopy;
- crystals are less intensely birefringent than monosodium urate crystals;
- more rod-shaped or rhomboid (as compared to the needles seen in gout) w/ birefringence on compensated polarized light microscopy;
- crystals may be small & difficult to see on nl light microscopy.
- Bipyramidal oxalate crystals
- can cause acute arthritis but are much less common and are usually seen in renal dialysis patients
Does the presence of crystal arthritis rule out septic arthritis?
Gout or 'pseudogout': how to differentiate crystal-induced arthropathies