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How is polarized light microscopy performed to definitively diagnose CPPD?
A drop of synovial fluid is placed on a clean microscope slide and covered with a cover slip. The slide is first examined under an ordinary light microscope, but the crystals are rarely visible. Therefore, a polarizer is typically needed to find CPP crystals, which appear as birefringent rhomboid or rectangular crystals with blunt or squared ends (Arthrocentesis and Synovial Fluid Analysis). For a definitive diagnosis, a first-order red compensator is applied. CPP crystals are referred to as weakly positively birefringent . This means that CPP crystals appear blue when viewed under polarized light, with the long axis of the crystal parallel to the direction of slow vibration of light in the compensator.
How the crystals look under polarized light?
The mnemonic ABC ( A ligned B lue C alcium) is useful: if the crystal is aligned with the red compensator and is blue, then it is CPP. CPP crystals lying with their long axes at right angles to the direction of slow vibration will appear yellow rather than blue. Observation of crystals inside a PMN, instead of floating free, helps to confirm that the CPP crystals are causing the arthritis.
