What investigations should be performed to confirm a diagnosis of CPPD in a patient with suspected pseudogout?

Investigations to confirm calcium pyrophosphate deposition disease (CPPD) in suspected pseudogout include:

• Synovial fluid analysis: The definitive diagnostic test is joint aspiration with microscopic examination of synovial fluid to identify calcium pyrophosphate crystals. These crystals are typically rhomboid-shaped and show weak positive birefringence under polarized light microscopy, distinguishing them from monosodium urate crystals seen in gout NICE NG219.
• Plain radiographs (X-rays) of the affected joint: Radiographic evidence of chondrocalcinosis, which is calcification in cartilage, supports the diagnosis of CPPD. This is a common finding in CPPD but not pathognomonic, so it should be interpreted alongside clinical and synovial fluid findings NICE NG219.
• Additional laboratory tests: While not diagnostic, serum calcium, magnesium, and phosphate levels may be assessed to identify metabolic conditions associated with CPPD, such as hypomagnesemia or hyperparathyroidism, which can predispose to crystal deposition Cadiou et al. 2022.
• Advanced imaging: In atypical or unclear cases, ultrasound or CT scanning can detect crystal deposits and cartilage calcification, but these are generally adjuncts rather than first-line investigations Tsui 2012.

Summary: The cornerstone for confirming CPPD in suspected pseudogout is synovial fluid analysis for calcium pyrophosphate crystals combined with radiographic evidence of chondrocalcinosis. Metabolic screening may help identify underlying causes contributing to crystal deposition Tsui 2012Cadiou et al. 2022 NICE NG219.