What will the synovial fluid analysis show in an elderly male patient with a 1-hour history of severe right knee pain, no history of falls or injuries, and a past medical history of hyperparathyroidism (overactive parathyroid gland) managed with bisphosphonates, who has had two similar episodes in the past year, and now presents with an arthritic, warm, and tender right knee, and significant hypertensity and tenderness in the vertebral musculature from T10 to L1, and radiography showing chondrocalcinosis (calcium deposits in cartilage)?

Synovial Fluid Analysis in CPPD Disease (Pseudogout)

The synovial fluid analysis will show calcium pyrophosphate dihydrate (CPPD) crystals that appear as rhomboid-shaped, weakly positively birefringent crystals under compensated polarized light microscopy. 1, 2

Clinical Context Supporting CPPD Diagnosis

This patient's presentation is classic for acute calcium pyrophosphate deposition (CPPD) disease, also known as pseudogout:

• Elderly male with recurrent acute monoarticular arthritis affecting the knee, with episodes resolving spontaneously within days—this pattern is pathognomonic for pseudogout 1, 2
• Radiographic chondrocalcinosis is the hallmark imaging finding that confirms CPPD crystal deposition in cartilage 3
• History of hyperparathyroidism is a well-established metabolic condition that promotes or causes chondrocalcinosis 1, 4

Expected Synovial Fluid Findings

Crystal Characteristics

• CPPD crystals appear as rhomboid or rod-shaped crystals that are weakly positively birefringent under compensated polarized light microscopy 1, 2, 5
• This contrasts with monosodium urate crystals in gout, which are needle-shaped and negatively birefringent 6

Additional Fluid Characteristics

• Inflammatory synovial fluid with elevated white blood cell count, typically 2,000-100,000 cells/mm³ with neutrophil predominance 1, 5
• The fluid analysis confirms the diagnosis when CPPD crystals are visualized in the context of radiographic chondrocalcinosis 3, 1

Critical Diagnostic Considerations

Mandatory Additional Testing

• Gram stain and culture must be performed even when CPPD crystals are identified, as crystal arthritis and septic arthritis can coexist 6
• This is particularly important in elderly patients with warm, erythematous joints where infection cannot be clinically excluded 3

Metabolic Workup

• Early-onset disease (before age 60) requires screening for metabolic conditions, particularly hemochromatosis, hyperparathyroidism, hypomagnesemia, and hypophosphatemia 1
• This patient's known hyperparathyroidism explains the CPPD deposition, as rapid drops in serum calcium after parathyroid treatment can precipitate acute pseudogout attacks 4

Common Diagnostic Pitfalls to Avoid

• Do not diagnose pseudogout based on chondrocalcinosis alone—synovial fluid crystal identification is essential for definitive diagnosis, as chondrocalcinosis can be asymptomatic in 10-15% of patients aged 65-75 years 3, 1
• Do not mistake pseudogout for septic arthritis or gout—always perform arthrocentesis when joint effusion is present to confirm crystal disease and exclude infection 7, 1
• Formalin dissolves CPPD crystals—any tissue or fluid sent for microscopic examination must be preserved in saline, not formalin 5