Diagnostic Sensitivity of Ultrasound vs Crystal Analysis for Gout

Though diagnosis of gout has traditionally been based on the presence of monosodium urate (MSU) crystals in synovial fluid, the use of musculoskeletal ultrasound is growing in both the research arena and the clinical setting. Research pertaining to the performance of this approach, however, have been limited by small sample sizes and other factors pertaining to study sites and patient variables.

In a paper published in Arthritis & Rheumatology, researchers from the University of Pennsylvania, Boston University School of Medicine, and numerous international institutions analyzed new evidence to determine the sensitivity and specificity of ultrasound for gout diagnosis, as well as patient and disease characteristics associated with a positive diagnostic scan. Their overarching goal was to elucidate the utility of ultrasound for gout diagnosis in clinical practice.

The authors examined data from the Study for Updated Gout Classification Criteria (SUGAR), an international, multi-center cross-sectional study of patients with at least one swollen joint and previously confirmed MSU crystals (via arthrocentesis or soft tissue aspiration) whose differential diagnosis included gout.

“The SUGAR study was performed to examine the components of gout that were most helpful for classifying gout and enrolled patients who could conceivably have gout but may or may not have had a previous diagnosis,” Alexis R. Ogdie-Beatty, MD, MSCE, a rheumatologist and assistant professor of medicine at the Hospital of the University of Pennsylvania, told Rheumatology Advisor.

In the current study, Dr Ogdie and colleagues focused on patients who had undergone ultrasound examination of the affected joints, which brought the sample size to a total of 416 cases and 408 control subjects who had tested negative for MSU crystals. The ultrasounds were performed by radiologists or rheumatologists who were blinded to previously obtained MSU results.

Based on the results, the researchers calculated the sensitivity, specificity, positive predictive values (PVPV), and negative predictive values (NPV) of the features of interest, which included double contour sign (DCS), tophus, and “snowstorm” appearance on ultrasound. “We hypothesized that duration of disease may impact the diagnosis of gout by ultrasound,” Dr Ogdie explained to Rheumatology Advisor.

The analysis revealed the following findings:

• Sensitivity for the presence of any one of the features was 76.9% and was higher in patients with a disease duration of ≥2 years, as well as those with suspected tophus on exam

• Specificity for the presence of any one of the features were 84.3%

• PPV for the presence of any one of the features was 83.3%

• NPV for the presence of any one of the features was 78.1% 

• Positive ultrasound findings were associated with suspected clinical tophus [odds ratio (OR) 4.77; 95% confidence interval (CI), 2.23-10.21], any abnormal plain film radiograph (OR 4.68; 95% CI, 2.68-8.17), and serum urate (OR 1.31; 95% CI, 1.06-1.62)

This study found that the specificity of double contour sign and tophi on ultrasound are high for gout regardless of the duration of disease,” Dr Ogdie indicated. “However, these ultrasound features may not be present in all patients with gout so a negative test doesn’t necessarily rule out the disease.”

Overall, while musculoskeletal ultrasound was found to have high specificity for gout diagnosis in this group of patients, the sensitivity was modest. “The specificity remained high for early disease and without clinical signs of tophi, the population in which ultrasound may be the most useful in establishing a diagnosis of gout,” the authors concluded

Summary and Clinical Applicability

The use of ultrasound for gout diagnosis was found to have high specificity and modest sensitivity. Duration of disease and certain disease features influence the performance of ultrasound for this application.

Limitations and Disclosures

• Possible introduction of selection bias
• Inability to control for variations in ultrasonographer training and specific ultrasound machines utilized

Multiple authors report speakers’ fees, consulting fees, or investigation grants from various companies and organizations.

Reference

Ogdie A, Taylor WJ, Neogi T, et al. Performance of ultrasound in the diagnosis of gout in a multi-center study: comparison with monosodium urate crystal analysis as the gold standard. Arthritis Rheumatol. 2016 Oct 16. doi: 10.1002/art.39959 [Epub ahead of print] 

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