Differentials
Pseudogout (calcium pyrophosphate deposition disease)
SIGNS / SYMPTOMS
Presentation may be identical to that of gout.[57]
Pseudogout is more likely to affect wrist and knee joints.
INVESTIGATIONS
Chondrocalcinosis (radiographic calcification of cartilage in certain joints) is usually present.
Ultrasound may help to differentiate calcium pyrophosphate deposition disease (CPPD) from gout. Calcium pyrophosphate deposits are found deeper in the cartilage and are less homogenous (lumpy-bumpy) than the superficial double contour sign seen in gout.
The definitive diagnosis is finding calcium pyrophosphate crystals in the synovial fluid. These are rhomboid-shaped, weakly positively birefringent crystals.
Septic arthritis
SIGNS / SYMPTOMS
Presentation may be identical to that of gout.[53][57]
Occurs in both sexes and at any age.
Risk factors for infection, such as intravenous drug use and immunocompromise, may be present.
INVESTIGATIONS
Synovial fluid microscopy and culture may be Gram positive and show growth.
Blood cultures may grow the causal bacteria.
Coexistence of crystals and infection in the joint is not uncommon.
Trauma
SIGNS / SYMPTOMS
A positive history is present.
Usually, there are fewer inflammatory signs, such as erythema or warmth, on joint examination than with gout.
INVESTIGATIONS
Synovial fluid is usually bloody and has no monosodium urate crystals.
Rheumatoid arthritis (RA)
SIGNS / SYMPTOMS
Chronic tophaceous and polyarticular gout may present like RA, and tophi can be misdiagnosed as rheumatoid nodules.
History of intermittent, acute, self-limited attacks of arthritis and podagra suggests gout.
RA and gout appear to be negatively correlated, as very few cases of coexistence have been reported.
INVESTIGATIONS
Associated with positive rheumatoid factor (RF) in 70% to 78% of cases; however, 30% of patients with gout have a positive RF.[78]
Anti-cyclic citrullinated peptide (anti-CCP) has high specificity, but low sensitivity, for RA. It may be useful in the early detection of patients who will have severe RA.[79]
Synovial fluid is inflammatory (WBC count >2000/mm³), but no monosodium urate crystals are found.
Reactive arthritis
SIGNS / SYMPTOMS
Recent infection with appropriate organism.
Oligoarthritis present.
Commonly affects weight-bearing joints.
May have tendon insertion inflammation and dactylitis (whole digit inflammation).
Conjunctivitis, urethritis, and stomatitis may be present.
INVESTIGATIONS
X-rays may show soft-tissue swelling.
Psoriatic arthritis
SIGNS / SYMPTOMS
Patients usually have a history of psoriasis.
Asymmetrical joint distribution.
Commonly affects the distal interphalangeal joints.
Presence of dactylitis.
INVESTIGATIONS
Typical radiographic findings include joint erosions, joint space narrowing, bony proliferation including periarticular and shaft periostitis, osteolysis including "pencil in cup" deformity and acro-osteolysis, ankylosis, spur formation, and spondylitis.[80]
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