Gout Diagnosis in Acute Monoarticular Arthritis
Joint aspiration with synovial fluid analysis demonstrating needle-shaped, negatively birefringent monosodium urate (MSU) crystals under polarized light microscopy is the definitive diagnostic test for gout, achieving sensitivity of 84% and specificity approaching 100%. 1, 2, 3
When Joint Aspiration Is Mandatory
Perform arthrocentesis whenever septic arthritis cannot be clinically excluded, as gout and bacterial infection can coexist in the same joint—in one series, 73% of patients meeting gout criteria had positive bacterial cultures, with two deaths and one amputation resulting from missed infections. 1
Aspiration is required when this is the patient's first suspected gout attack, when the clinical presentation is atypical (polyarticular involvement, unusual joint distribution, systemic symptoms including fever), or when diagnosis remains uncertain after clinical assessment. 1, 2
Even when MSU crystals are identified, Gram stain and culture must still be performed (sensitivity 76%, specificity 96% for septic arthritis), because crystal arthritis and infection frequently coexist. 1, 3
Clinical Diagnosis When Aspiration Is Not Feasible
When joint aspiration cannot be performed, validated clinical algorithms combined with imaging achieve diagnostic accuracy exceeding 80%. 1, 2
High-Value Clinical Features
Podagra (first metatarsophalangeal joint involvement) has the highest diagnostic value with sensitivity 96%, specificity 97%, and likelihood ratio of 30.64. 1
Rapid pain onset reaching maximum intensity within 6–12 hours is highly characteristic of crystal inflammation and strongly favors gout over other arthritides. 1, 4
Erythema overlying the affected joint combined with rapid-onset severe pain substantially increases diagnostic probability. 1, 4
History of prior similar attacks (recurrent pattern) is a strong supporting feature. 1
Male gender, cardiovascular comorbidities, and diuretic use increase pre-test probability. 1
ACR/EULAR Classification Criteria
The 2015 ACR/EULAR criteria use a hierarchical scoring system across clinical, laboratory, and imaging domains; a score ≥8 points is diagnostic for gout with sensitivity 92% and specificity 89% (including imaging) or sensitivity 85% and specificity 78% (excluding imaging). 1, 2
Serum Uric Acid: Critical Limitations
Hyperuricemia alone cannot diagnose gout—only 22% of asymptomatic individuals with serum uric acid >9 mg/dL develop gout within 5 years, and up to 80% of patients with hyperuricemia never develop gout. 1
Approximately 10% of patients with acute gout have normal serum uric acid levels during an attack because uric acid behaves as a negative acute-phase reactant during inflammation. 1, 3
Serum uric acid has only 53–61% specificity for gout diagnosis but is essential for long-term management decisions and urate-lowering therapy titration. 1
Imaging Algorithm
First-Line: Ultrasound
Ultrasound is the initial advanced imaging modality when joint aspiration is not feasible or clinical diagnosis is uncertain. 1, 2
The "double contour sign" (hyperechoic band over hyaline cartilage) has sensitivity 74% and specificity 88% for detecting MSU crystal deposition, with a likelihood ratio of 13.6. 1, 5
Always include bilateral first metatarsophalangeal joints in ultrasound examination, even if asymptomatic, as this is the disease-specific target site. 1
Ultrasound sensitivity is significantly lower in early disease (<2 years duration): 50% versus 63% in late disease. 1
Tophi appear as hyperechoic masses with a "wet clumps of sugar" appearance, often surrounded by an anechoic halo (sensitivity 65%, specificity 80%). 1
Second-Line: Dual-Energy CT (DECT)
DECT should be considered when ultrasound is negative but clinical suspicion remains high, particularly in patients with disease duration >2 years. 1, 2
DECT detects MSU crystal deposition with sensitivity 85–100% and specificity 83–92%, providing color-coded images that distinguish urate deposits from other soft tissue. 1, 6, 5
DECT is less sensitive in patients with shorter disease duration or fewer prior flares. 1
When characteristic MSU crystal deposition is identified on ultrasound (double contour sign or tophi) or DECT, synovial fluid analysis is not needed to confirm diagnosis. 1
Limited Role of Plain Radiography
- Plain radiographs are useful only for assessing chronic structural damage (erosions with overhanging edges in late-stage disease) or excluding alternative diagnoses; they should not be the primary diagnostic modality for acute gout as they are frequently normal in early disease. 1, 6, 7
Risk Factor and Comorbidity Assessment
Systematically evaluate modifiable risk factors and associated comorbidities that influence both diagnosis and management: 1, 2
| Risk Factor/Comorbidity | Relative Risk |
|---|---|
| Chronic kidney disease | 4.95 |
| Hypertension | 3.93 |
| Obesity | 3.81 |
| Diuretic use | 1.72 |
Screen for renal impairment, ischemic heart disease, heart failure, diabetes mellitus, and dyslipidemia. 1
Assess dietary factors: alcohol consumption (especially beer), meat and shellfish intake, and high-fructose corn syrup consumption. 1
Review medications: thiazide and loop diuretics, low-dose aspirin, cyclosporine, and tacrolimus all decrease renal uric acid excretion. 1
Practical Diagnostic Algorithm
If septic arthritis cannot be excluded or this is the first attack: Perform joint aspiration with crystal analysis, Gram stain, and culture. 1, 2
If classic podagra with typical features (rapid onset, erythema, no fever, multiple risk factors) and infection is unlikely: Diagnose gout clinically using validated criteria. 1
If clinical diagnosis is uncertain and aspiration is not feasible: Obtain ultrasound of symptomatic joint(s) plus bilateral first MTP joints. 1, 2
If ultrasound is negative but suspicion remains high (especially disease >2 years): Consider DECT to detect MSU crystal deposition. 1, 2
If all testing is inconclusive: Refer to rheumatology for expert evaluation. 2
Common Pitfalls to Avoid
Never diagnose gout based solely on hyperuricemia—normal uric acid does not exclude gout, and elevated levels do not confirm it. 1
Never assume fever is merely a gout flare manifestation—it is a red-flag sign requiring immediate evaluation for coexistent septic arthritis. 1
Do not rely on imaging alone—patient history, physical examination, and laboratory findings must be integrated into diagnostic assessment. 1
Ultrasound and DECT require trained operators; interpretation by inexperienced personnel significantly reduces diagnostic accuracy. 1
MSU crystals can be detected in approximately 70% of asymptomatic joints in patients with established gout, even in joints never previously inflamed, so crystal presence alone does not confirm an acute flare. 3