An elderly patient with a swollen knee and prior effusion has synovial fluid demonstrating calcium pyrophosphate crystals. What is the most likely diagnosis?

An elderly patient with a swollen knee and prior effusion has synovial fluid demonstrating calcium pyrophosphate crystals. What is the most likely diagnosis?

• A. Septic arthritis
• B. Acute synovitis
• C. Hemarthrosis
• D. Pseudogout

Answer: (D)

The key idea is recognizing calcium pyrophosphate crystal deposition in the joint fluid as pseudogout. In older adults, a swollen knee with a history of prior effusions is a classic setting for CPPD disease, and finding calcium pyrophosphate crystals in the synovial fluid clinches the diagnosis. These crystals are typically rhomboid-shaped and show weak positive birefringence under polarized light, which differentiates them from gout crystals (needle-shaped and negatively birefringent). This finding makes septic arthritis less likely in the absence of fever, systemic signs, or purulent fluid, and it fits better than a generic acute synovitis or hemarthrosis, which would not show calcium pyrophosphate crystals. Thus, the most likely diagnosis is pseudogout.

The key idea is recognizing calcium pyrophosphate crystal deposition in the joint fluid as pseudogout. In older adults, a swollen knee with a history of prior effusions is a classic setting for CPPD disease, and finding calcium pyrophosphate crystals in the synovial fluid clinches the diagnosis. These crystals are typically rhomboid-shaped and show weak positive birefringence under polarized light, which differentiates them from gout crystals (needle-shaped and negatively birefringent).

This finding makes septic arthritis less likely in the absence of fever, systemic signs, or purulent fluid, and it fits better than a generic acute synovitis or hemarthrosis, which would not show calcium pyrophosphate crystals. Thus, the most likely diagnosis is pseudogout.