Ultrasound Can Guide Accurate Gout Diagnosis
Interview with Alexis Ogdie, MD, MSCE
Traditionally, the gold standard for diagnosing gout has been to extract monosodium urate (MSU) crystals from the synovial fluid of the patient.1 However, according to a recent study published in Arthritis & Rheumatology,2 ultrasound may provide a less invasive diagnostic option for patients with gout.
Ultrasound features for MSU crystal disposition showed a high specificity and predictive value for making a reliable gout diagnosis—even in cases where there are no clinical signs of tophi, noted Alexis Ogdie, MD, MSCE, from the Perelman School of Medicine, at the University of Pennsylvania in Philadelphia, Pennsylvania.
Aspiration vs Ultrasound
Reliable and thoroughly vetted, needle arthrocentesis or joint aspiration can be a very painful experience for the patient, especially given the skin surrounding the gout-inflamed joint is typically very sensitive. In addition, not all doctors are familiar with the technique.
“While arthrocentesis is a common procedure in rheumatology and rheumatologists are comfortable with this procedure,” not all physicians are as comfortable with the procedure, including primary care physicians who do not always see gout cases in their daily practice, Dr. Ogdie told Practical Pain Management.
“Additionally, sometimes the affected joint isn’t amenable to arthrocentesis or the joint effusion has mostly resolved. Thus, one of the goals of this group was to develop classification criteria for gout that would accurately reflect the gold standard (MSU crystals in the joint fluid). In doing so, ultrasound was identified as a valuable diagnostic test for gout,” Dr. Ogdie said.
Musculoskeletal ultrasound is fast becoming a more recognized method for establishing a gout diagnosis.3-9 Using ultrasound, doctors can detect various indicative signs, including:
- Hyperechoic irregular enhancement of the articular surface of the hyaline cartilage
- Hyperechoic aggregates that indicate the presence of tophi within the joint or along the tendons
- A snowstorm-like appearance that indicates floating hyperechoic foci within the joint space
Ultrasound "has been studied in a handful of other studies, but many of those studies didn’t include MSU as the gold standard and many of the patients had long-standing gout,” Dr. Ogdie said. In addition, previous studies have included expert practitioners in musculoskeletal ultrasound. This conceivably could have skewed past data, not really reflecting the true clinical accuracy of musculoskeletal ultrasound when used in a common practice setting.
A Large Multi-Center, Multi-Nation Study
The Study for Updated Gout Classification Criteria (SUGAR), used data from a large, international, multi-center cross-sectional study. Featuring 982 total patients, 824 of which received ultrasound (416 gout cases and 408 controls).
Other patient characteristics (of those in the ultrasound analysis) included:
- Mean ages for cases and controls were 60.2 (SD 14.6) and 59.5 (SD 16.0), respectively
- Sexes for cases and controls were 87% and 54% were male, respectively
- Percentage of cases and controls who had more than 5 episodes of joint “flares” was 69% and 49%, respectively
To judge ultrasound’s efficacy against synovial joint fluid extraction, all patients also underwent arthrocentesis.
Researchers found ultrasound independently contributed to identifying gout in patients, showing an odds ratio of 7.2, suggesting musculoskeletal ultrasound can be clinically useful at identifying gout or non-gout in patients, with a similar consistency to the identification of tophi,10 the authors noted.
For any one of the clinical features of gout, MSU showed high percentages for sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV), at 76.9%, 84.3%, 83.3%, and 78.1%, respectively.
Musculoskeletal ultrasound also maintained a high specificity (88% to 96%) and positive predictive value (93% to 97%) for establishing gout when performed on patients with calcium pyrophosphate deposition (CPPD) disease and gout, which could allow for differentiation between gout and CPPD. A positive ultrasound also appeared to be associated with a higher urate burden in general.
While musculoskeletal ultrasound did show a high positivity for gout diagnosis compared to the gold standard of arthrocentesis, ultrasound’s sensitivity and NPV decreased as the features of gout increased in patient cases. So while ultrasound has a high predictive value, “among patients with gout, only approximately 50% to 60% will have a positive ultrasound,” Dr. Ogdie told Practical Pain Management.
This could be a relevant clinical concern for practitioners, considering establishing a reliable gout diagnosis is paramount to patient outcomes. And given the condition’s symptoms can be similar to other conditions, including various types of arthritis and joint infections, treatment decisions must be made on a reliable diagnosis.
Indeed, control subjects showed a variety of other diagnoses that could have shared some appearances to gout, including:
- CPPD (n=98)
- osteoarthritis (n=63)
- rheumatoid arthritis (n=59)
- spondyloarthropathies (n=60)
- undifferentiated inflammatory arthritis (n=47)
- septic arthritis (n=7)
- systemic lupus erythematosus (n=5)
- clinically suspected gout (n=41)
- other (n=28)
But as a pain-free option, musculoskeletal ultrasound appears to be a beneficial alternative, considering many practitioners may not have arthrocentesis as a clinical option. Also, some doctors may already have obtained a MSU-positivity test from a patient but want to assess a joint from which the arthrocentesis was not performed. In those cases, ultrasound could be a convenient work-around to take advantage of, Dr. Ogdie told Practical Pain Management.
Practitioners just may want to be aware of ultrasound’s modest sensitivity for patients with early disease (≤2 years) without suspected tophi on examination. Granted, the data suggests ultrasound functions best in patients who could be more easily diagnosed without imaging (tophi at examination), but ultrasound still performs well with patients who do not show obvious signs of gout upon examination, the authors noted.