Most needle-shaped calcium pyrophosphate crystals lack birefringence

CPP crystals can be polymorphic (rhomboidal, parallelepiped: R/P), but some look like needles and could be taken as MSU under the bright field microscope. Birefringence of CPP crystals is weaker or absent compared with MSU crystals, but we aim to evaluate whether the grade of birefringence varies regarding the shape of the CPP crystal.

SF samples from patients with demonstrated acute CPP crystal arthritis were analysed by two observers, using a simple polarized light microscope equipped with two viewing stations. The analysis was performed simultaneously but in a blinded manner. Shape (needles or R/P) and the intensity of birefringence (absent, weak, moderate or MSU-like) were registered. χ2 trend test was used to evaluate the distribution of birefringence regarding the crystal shape.

Two-hundred and fifty CPP crystals from 25 SF samples were analysed, well balanced between R/P and needles. The intensity of birefringence significantly differs between R/P or needles in the registries of both observers. R/P most often showed any grade of birefringence compared with needles, while no cases of MSU-like birefringence were found in acicular crystals. Both observers showed high agreement both in crystal shape and in intensity of birefringence.

CPP crystals birefringence varies according to shape. Needle-shaped CPP crystals did not show strong birefringence, thus reinforcing the value of examining the samples with both ordinary and simple polarized light microscopes in differentiating them from MSU.