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In
gout, during attacks of acute arthritis, monosodium urate
crystals (MSU) can be demonstrated in synovial fluid of
about 90-95% of patients. Between these attacks, MSU
crystals can be demonstrated in about 70% of patients.
During the acute episodes, the majority of crystals are
intracellular, within neutrophils or macrophages; however,
between attacks the majority of crystals are
extracellular. In a few patients with acute gout, even
careful examination with compensated polarized light
fails to reveal MSU crystals. Possible reasons for this
failure may include crystals trapped within a joint and
dissolution of the crystals into the synovial fluid.
Occasionally, a 30-45 minute search may be required to
find one or two MSU crystals in a patient with gouty
arthritis. In such cases, there is an obvious need for
the attending physician to communicate his clinical
impression to the pathologist, requesting a prolonged
study of the specimen.
The laboratory report should note whether MSU crystals
are lying free in the synovial fluid or have been
ingested by leukocytes. Phagocytosis of crystals suggests
that these are responsible for acute gouty arthritis. If
only extracellular crystals are found, it is unlikely
that these are producing acute symptoms.
When in doubt about the presence of urate crystals, a
biochemical determination of uric acid in joint fluid may
be of help. Joint fluid uric acid may be elevated (above
serum normal range) in patients with gout, confirming the presence of urate
crystals. However, the finding of
normal or low levels of uric acid in a joint fluid (as
compared with serum normal range) does not rule out the presence
of these crystals.
In cases of pseudogout (rheumatoid arthritis or
osteoarthritis), clinical symptoms may mimic gout. The
diagnosis in these cases requires the demonstration of
calcium pyrophosphate crystals. As in gout, the crystals
may be either intracellular or extracellular but they
show an inverse color pattern under polarized light using
a red filter. Vertically oriented pyrophosphate crystals
look blue when observed with the red vertical filter and
yellow with the horizontal red filter. Acid uric levels
of synovial fluid are normal or low in these cases.
NOTES AND PRECAUTIONS:
1. Artifacts that may be mistaken for crystals include
collagen fibers, pieces of cartilage, scratches and dirt
on the glass slide and phagocytosis of metal or plastic
materials in joints containing prostheses.
2. Crystal artifacts may be found in samples obtained
with powder EDTA or oxalate. These powdered forms of
anticoagulants must be avoided.
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