Calcium Pyrophosphate Deposition Disease (CPPD/Pseudogout)
The most likely diagnosis is D - Calcium Pyrophosphate Deposition Disease (CPPD), also known as pseudogout, given the recurrent monoarticular knee pain over 3 months with normal uric acid levels and normal WBC count, which effectively rules out both gout and septic arthritis. 1
Why Normal Uric Acid Rules Out Gout
- Normal serum uric acid levels essentially rule out gout in the clinical context of recurrent monoarticular knee pain, according to the European League Against Rheumatism 1
- The absence of hyperuricemia has a markedly low likelihood ratio for gout diagnosis, making it very unlikely 1
- While serum uric acid can be normal during acute gout attacks, this applies to acute presentations, not a 3-month history of recurrent episodes 2
- Hyperuricemia alone has only 53-61% specificity for gout, but its absence is highly predictive against gout 1
Why CPPD (Pseudogout) Fits Best
- CPPD is the second most common crystal arthropathy and frequently affects the knee joint 3
- CPPD characteristically presents with recurrent episodes of monoarticular arthritis without systemic manifestations, exactly matching this clinical picture 4
- The knee is one of the most commonly affected joints in CPPD, along with wrists and ankles 4
- Risk factors for CPPD include advanced age, osteoarthritis, hyperparathyroidism, hemochromatosis, and hypomagnesemia 3
Why Not Reactive or Septic Arthritis
- Reactive arthritis (B) is excluded by the absence of recent illness history and the 3-month duration of recurrent episodes 2
- Septic arthritis (C) is ruled out by normal WBC count, absence of fever, and absence of systemic manifestations 3
- Normal inflammatory markers and lack of acute systemic signs make infection highly unlikely 3
Definitive Diagnostic Approach
- Joint aspiration with synovial fluid analysis is mandatory to confirm the diagnosis and exclude other conditions 1
- CPPD diagnosis requires identification of calcium pyrophosphate crystals in synovial fluid—these appear as rhomboid-shaped, weakly positively birefringent crystals under polarized microscopy 3
- Gram stain and culture must still be performed even when crystals are identified, as crystal arthritis and septic arthritis can coexist 1, 3
Imaging Considerations
- Look for chondrocalcinosis on plain radiographs of the knee, which appears as linear calcifications in the cartilage 3
- Conventional radiography and ultrasound are the recommended initial imaging modalities for CPPD diagnosis 3
- Radiographic evidence of chondrocalcinosis supports CPPD but is not always present, particularly in early disease 5
Common Pitfall to Avoid
- Do not assume gout based on recurrent monoarticular arthritis alone—normal uric acid makes this diagnosis extremely unlikely, and CPPD is a common mimic that requires crystal identification for definitive diagnosis 1, 4
- Crystal deposits in joints can be destructive as well as painful, so early identification and management prevents degenerative disease 4