Synovial fluid crystal analysis — SCE Rheumatology MCQ

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ModerateMusculoskeletal InvestigationsSynovial fluid crystal analysisSCE Rheumatology

A 59-year-old woman has acute wrist monoarthritis. Synovial fluid microscopy shows rhomboid crystals with weak positive birefringence under polarised light. Culture is negative after initial incubation. Wrist radiograph shows triangular fibrocartilage calcification. What is the most likely diagnosis?

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Correct answer: B — Calcium pyrophosphate crystal arthritis

Explanation lettering: E = shown as C · C = shown as D · D = shown as E

Calcium pyrophosphate crystal arthritis is best because weakly positively birefringent rhomboid crystals and chondrocalcinosis identify CPP crystal arthritis. A is less suitable because sepsis is less likely with crystals and negative culture but should be excluded clinically; C is less suitable because gout has needle-shaped negatively birefringent crystals; D is less suitable because RA does not cause CPP crystals; E is less suitable because basic calcium phosphate crystals are not seen as birefringent rhomboids. Clinical pearl: the shape and birefringence of crystals are common SCE discriminators.

Reference: Oxford Textbook of Rheumatology; BSR gout guideline