Background Calcium pyrophosphate (CPP) crystals can be polymorphic (rhomboidal, parallelepiped), but some look like needles and could be taken as monosodium urate (MSU) under the ordinary light microscope. When simple polarized-light is applied, only around 20% of calcium pyrophosphate (CPP) crystals shine , but weaker if compared to monosodium urate (MSU) crystals. To date, whether the intensity of birefringence varies regarding the shape of the crystal has not been explored.
Methods Cross-sectional study. Synovial fluid (SF) samples from consecutive patients with demonstrated acute CPP crystal arthritis were analysed by two observers, using a simple (non compensated) polarized light microscope equipped with two viewing stations. The analysis was performed simultaneously but in a blinded manner. The first 10 consecutive PFC crystals of each SF detected with bright light at 600X were assessed, being pointed with the microscope arrow and observed for birefringence under crossed polarized filters. The following variables were registered: a) shape (needle or rhomb- parallelepiped), and b) intensity of birefringence (absent, weak, or MSU-like); previous to the observation the microscope light, diaphragm and condenser height were adjusted to clearly show the birefringence using a MSU crystal preparation as standard. Previously, both observers using unblinded samples had reached a consensus on shape and birefringence intensity. Chi-square test was used to evaluate the distribution of birefringence regarding the crystal shape; inter-observer agreements in shape and birefringence were assed using weighted kappa test.
Results 120 crystals from 12 SF samples were analysed, 61 (50.8%) were classified as needles and 59 (49.2%) as rhombi-parallelepipeds. The intensity of birefringence was graded as absent in 52 (43.3%) crystals, weak in 66 (55.0%), and MSU-like in only 2 (1.7%). Birefringence was found to differ regarding the crystal shape (Figure, p<0.001), rhombi-parallelepipeds most often showed some birefringence compared to needles. In no cases, birefringence of needles was graded as MSU-like. Inter-observer agreement was very high both for crystal shape (kappa 0.97, 95%CI 0.92-1.00) and for birefringence (kappa 0.71, 95%CI 0.60-0.82).
Conclusions 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 polarised light microscope in differentiating them from MSU.
Ann Rheum Dis. 58:582.
Disclosure of Interest None declared