Synovial fluids can contain a number of crystals and other particulate matter. Some of these, particularly monosodium urate monohydrate (MSUM) and calcium pyrophosphate dihydrate (CPPD) crystals, are pathogenic; others, including cholesterol and other lipid particles and the basic calcium phosphates (BCPs), including apatites, are of doubtful significance. Table 1 lists the main forms of particulate matter that have been identified in synovial fluids. View this table: Table 1 Intrinsic and extrinsic crystals and particles detected in synovial fluid In the case of MSUM and CPPD crystals it is clear that the identification of these crystals in a synovial fluid that also has a high polymorphonuclear cell count (indicative of acute inflammation) is the only certain way to diagnose an attack of gout or pseudogout respectively.1 Furthermore, this is one of the few tests that has been shown to change clinical practice in rheumatology,2 and it is apparent that the consequences of getting the diagnosis wrong can be severe.3 Therefore, accurate identification of MSUM and CPPD is important. 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. 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.5 6 In clinical practice we need a relatively simple, affordable technique, with a reasonable degree of sensitivity and specificity. 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 …