CPPD Crystal Identification — SCE Rheumatology MCQ

Instant feedback + full explanation. One question, done properly.

EasyCrystal ArthropathyCPPD Crystal IdentificationSCE Rheumatology

A 50-year-old woman presents with acute monoarthritis. Synovial fluid is aspirated before any intra-articular treatment. Compensated polarised light microscopy shows intracellular rhomboid and short rod-shaped crystals that are weakly positively birefringent, appearing blue when their long axis is parallel to the compensator axis. Which crystals are present?

Educational content. Not a substitute for clinical judgement or local policy.

Reveal the answer and explanation

Correct answer: A — Calcium pyrophosphate crystals

The crystals are calcium pyrophosphate crystals. Their typical appearance is rhomboid or short rod-shaped with weak positive birefringence: blue when parallel and yellow when perpendicular to the compensator axis. In this clinical setting they support acute CPP crystal arthritis. Monosodium urate crystals are usually needle-shaped and strongly negatively birefringent, appearing yellow when parallel. Basic calcium phosphate crystals are generally too small to be identified by routine compensated polarised microscopy. Cholesterol crystals are larger, plate-like structures with characteristic notched corners. Injected corticosteroid preparations can produce strongly birefringent crystals, but aspiration occurred before intra-articular treatment. Identification of CPP crystals does not by itself exclude concurrent septic arthritis, so appropriate microscopy, Gram stain and culture remain necessary when infection is clinically possible.

Reference: The Persistency and Visibility of Synovial Fluid Crystals After at Least 10 Days in Refrigerator by Light and Polarized Microscopy, 2023. https://pubmed.ncbi.nlm.nih.gov/37525737/