There is evidence in support of each of these contentions. The lack of sensitivity is largely attributable to the fact that crystals may be either too small (individually) or too sparse (low concentration) to be identified. There are a number of reasons for the problem. Why is polarised light microscopy unreliable? However, we have yet to be convinced that the identification of apatite or other BCP particles in the synovial fluid ever should or does affect patient management, which renders it irrelevant to clinical practice. But polarised light microscopy is not a crystallographic technique (and therefore cannot permit certain characterisation), and it is virtually useless for the identification of basic calcium phosphates (BCP). Polarised light microscopy remains the only possibility that comes anywhere near fulfilling these needs: it is available in most hospitals and is relatively inexpensive in addition, as MSUM and CPPD crystals can be distinguished through their different polarisation properties, it should, in theory, be reasonably reliable for these two crystal types (figs 1 and 2). 5, 6 In clinical practice we need a relatively simple, affordable technique, with a reasonable degree of sensitivity and specificity. They range from the very simple, like Garrod’s famous “string test”, 4 to the furiously complex, such as laser microscopy or atomic force microscopy. There are a large number of techniques that can be used to identify crystals that can be found in synovial fluid, nearly all of which rely on microscopy of one sort or another because of the small size of the individual particles.
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