Ultrasound examination of patients with at least one swollen joint or a subcutaneous nodule detected gout with a sensitivity of 76.9% and specificity of 84.3% in an international, multicenter study.
“Ultrasound is a good test for gout,” said study lead author Alexis R. Ogdie-Beatty, MD, of the University of Pennsylvania, Philadelphia. “It has high specificity, even among patients with early disease. The sensitivity was lower so it doesn’t pick up all cases of gout, although it was higher in people with a longer duration of disease.”
Dr. Alexis R. Ogdie-Beatty
But gout remains difficult to diagnose because of the many similar types of inflammatory arthritis. A 2016 report from the Agency for Healthcare Research and Quality says proper diagnosis is a “major challenge,” especially in primary care and urgent/emergency settings.
Monosodium urate (MSU) crystal analysis via joint aspiration is considered the “gold standard” of gout diagnostic tools, but the agency report notes that it “can be technically difficult to perform and painful to the patient.”
There are other challenges. “Sometimes we see people between flares, or it’s already started to improve so there’s not enough fluid to perform an arthrocentesis,” Dr. Ogdie-Beatty said in an interview. “Additionally, while rheumatologists are generally very good at joint aspirations, many primary care physicians are not specifically trained for this. Thus, an imaging study can be helpful in these cases.”
The new study (Arthritis Rheumatol. 2016 Oct 16. doi: 10.1002/art.39959) follows up on findings from the multicenter, cross-sectional Study for Updated Gout Classification Criteria, which independently linked ultrasound analysis to diagnosis of gout with an odds ratio of 7.2 (Arthritis Care Res. 2015 Sep;67[9]:1304-15). One goal of the new study is to determine specificity and sensitivity of ultrasound exams.
The researchers examined data from the previous study, which enrolled consecutive patients in clinical practice settings across 25 countries who had at least one swollen joint or a subcutaneous nodule in whom gout was on the differential diagnosis. They underwent ultrasound examinations (most commonly of the knees, metatarsophalangeal joints, and ankles) and were deemed to have a “positive test” if at least one of three signs appeared: a “double contour” sign, tophus, or a “snowstorm” appearance. However, the study did not require a specific ultrasound scanning protocol or training.
MSU crystal examinations confirmed which of the subjects actually had gout. In total, ultrasound examinations were performed on 824 patients, of whom 416 were confirmed to have gout (mean age, 60, and 87% male), and the other 408 did not have gout (mean age, 60, and 54% male).
The researchers found that the sensitivity of the ultrasound exams was 76.9%, meaning they correctly identified patients with gout just over three-quarters of the time. Dr. Ogdie-Beatty referred to this as “moderate” sensitivity. The researchers found that the sensitivity was highest in patients who’d had disease for 2 or more years and in those who didn’t show clinical signs of tophi.
The specificity for these signs – the percentage of the time that ultrasound exams correctly identified patients without gout – was 84.3%. The positive and negative predictive values were 83.3% and 78.1%, respectively.
Dr. Ogdie-Beatty said the cost of ultrasound examinations is variable, although insurance covers it for trained providers. It’s unclear how commonly ultrasound examinations are used to detect gout, she said, but “many rheumatologists now have ultrasound as a part of their practice, and ultrasound training during fellowship has become important.”
Dr. Ogdie-Beatty reported receiving consulting fees from Novartis, and her coauthors reported various financial relationships with industry. The study was supported by various National Institutes of Health grants to several authors, as well as the American College of Rheumatology, the European League Against Rheumatism, and various arthritis-related organizations.