Arthrocentesis, or removing synovial fluid from the affected joint, is performed to test the synovial fluid for the calcium pyrophosphate crystals that are present in CPPD. When stained with H&E stain, calcium pyrophosphate crystals appears deeply blue ("basophilic"). However, CPP crystals are much better known for their rhomboid shape and weak positive Birefringence on polarized light microscopy, and this method remains the most reliable method of identifying the crystals under the microscope. However, even this method suffers from poor sensitivity, specificity, and inter-operator agreement.