Diagnosing Gouty Arthritis
Joint aspiration with synovial fluid analysis showing needle-shaped, negatively birefringent monosodium urate (MSU) crystals under polarized light microscopy is the definitive diagnostic test for gout, but when this is not feasible, clinical diagnosis using validated criteria combined with ultrasound imaging provides diagnostic accuracy exceeding 80%. 1, 2, 3
When Joint Aspiration Is Mandatory
Arthrocentesis must be performed in the following situations:
- Septic arthritis cannot be clinically excluded – gout and infection can coexist, making aspiration non-negotiable 1, 2
- First suspected gout attack in a patient's lifetime 2, 3
- Atypical clinical presentation (polyarticular involvement, unusual joint distribution, systemic symptoms) 1, 2
- Always perform Gram stain and culture even when MSU crystals are identified, as crystal arthritis and septic arthritis may coexist 2, 3
The aspiration should only be performed if: an experienced clinician can minimize infection risk, a polarizing microscope with trained operator is available, and the joint can be aspirated without substantial patient discomfort 1
When Clinical Diagnosis Alone Is Acceptable
You can confidently diagnose gout clinically without aspiration when the patient presents with classic podagra (first metatarsophalangeal joint involvement), rapid onset of maximum pain within 24 hours, no fever or systemic signs suggesting infection, and multiple characteristic risk factors. 1, 2
The most accurate clinical diagnostic algorithm includes these features (achieving 85% sensitivity and specificity): 2, 4
- Male sex
- Previous patient-reported similar arthritis attack
- Onset reaching maximum intensity within 1 day
- Joint redness (erythema over affected joint)
- First metatarsophalangeal joint involvement
- Hypertension or cardiovascular disease present
- Serum uric acid >5.88 mg/dL (>350 µmol/L)
Critical pitfall: The ACR/EULAR classification criteria achieve 92% sensitivity and 89% specificity with imaging, but the relatively low specificity raises the risk of missing septic arthritis, which presents similarly 2
Serum Uric Acid Measurement: Essential But Not Diagnostic
Measure serum uric acid in all suspected cases, but never diagnose gout based on hyperuricemia alone and never exclude gout based on normal levels. 2, 3
Key limitations to understand:
- Approximately 10% of patients with acute gout have normal serum uric acid during the attack (uric acid behaves as a negative acute phase reactant during inflammation) 2
- Only 22% of asymptomatic individuals with serum uric acid >9 mg/dL develop gout within 5 years 2
- Hyperuricemia alone has only 53-61% specificity for gout diagnosis 2
- However, normal serum uric acid levels essentially rule out gout in the context of recurrent monoarticular arthritis 2
Imaging: First-Line Ultrasound, Second-Line DECT
When to Use Ultrasound
Ultrasound should be the initial imaging modality when joint aspiration is not feasible or when clinical diagnosis remains uncertain. 2, 3
- The "double contour sign" on cartilage surfaces has 74% sensitivity and 88% specificity 2, 3
- Always scan the first metatarsophalangeal joint bilaterally, even if asymptomatic, as this is the disease-specific target site 2
- Ultrasound performs better in late disease (>2 years duration) with 63% sensitivity compared to 50% in early disease 2
- Tophi appear as hyperechoic masses with "wet clumps of sugar" appearance, detected with 65% sensitivity and 80% specificity 2
When to Use 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. 2, 3
- Sensitivity 85-100% and specificity 83-92% for detecting MSU crystal deposition 2, 3
- More expensive and involves radiation exposure, making it second-line 2
- Less sensitive in patients with shorter disease duration (<2 years) 2
- When characteristic MSU crystal deposition is identified on ultrasound or DECT, synovial fluid analysis is not needed to confirm diagnosis 2
Plain Radiography: Limited Role
Plain radiographs are useful only for assessing chronic structural damage or excluding alternative diagnoses and should not be used as the primary diagnostic modality 2, 3
Risk Factor and Comorbidity Assessment
Systematically screen for these modifiable risk factors and comorbidities: 2, 3
| Risk Factor/Comorbidity | Relative Risk |
|---|---|
| Chronic kidney disease | 4.95 |
| Hypertension | 3.93 |
| Obesity | 3.81 |
| Diuretic use | 1.72 |
- Medications: thiazide diuretics, loop diuretics, low-dose aspirin, cyclosporine, tacrolimus
- Alcohol consumption (especially beer)
- Diet high in meat, shellfish, and high-fructose corn syrup
- Ischemic heart disease, heart failure, diabetes, dyslipidemia
Practical Diagnostic Algorithm
If septic arthritis cannot be excluded or first attack: Perform joint aspiration with crystal analysis, Gram stain, and culture 1, 2, 3
If classic podagra with typical features and no infection concern: Diagnose clinically using validated algorithm (male sex, previous attack, onset <1 day, redness, MTP1 involvement, hypertension/CVD, uric acid >5.88 mg/dL) 2, 4
If clinical diagnosis uncertain and aspiration not feasible: Obtain ultrasound of symptomatic joints plus bilateral MTP1 joints 2, 3
If ultrasound negative but suspicion remains high (especially if disease >2 years): Consider DECT 2, 3
If all testing inconclusive: Refer to rheumatology for expert evaluation 3
Common Pitfalls to Avoid
- Never diagnose gout on hyperuricemia alone – up to 80% of patients with hyperuricemia never develop gout 2
- Never exclude gout based on normal uric acid during acute attack – 10% of acute gout patients have normal levels 2
- Never skip Gram stain and culture when crystals are found – coexistent infection occurs 2, 3
- Do not rely on imaging alone – integrate patient history, physical examination, and laboratory findings 2
- Ultrasound sensitivity is significantly lower in early disease (<2 years), with only 50% sensitivity for double contour sign 2