Criteria for sonographic diagnosis of monosodium urate (MSU) crystal deposition have been developed, but the inter-reader reproducibility of this modality is not well-established. We therefore assessed agreement using a systematic approach.
50 male subjects ages 55-85 were recruited during primary care visits to an urban VA hospital, and were assessed by musculoskeletal ultrasound (MSK-US) of the knees and 1st metatarsophalangeal (MTP) joints to evaluate for the “double contour” sign and tophi as evidence of MSU crystal deposition. Images were read by two blinded rheumatologists trained in MSK-US, and degree of concordance was determined for individual patients, total joints, femoral articular cartilage (FAC) and 1st MTP joints. Patients were further categorized into three diagnostic groups: gout, asymptomatic hyperuricemia (AH) (no gout, serum uric acid (UA) ≥ 6.9 mg/dL) and controls (no gout, UA ≤ 6.8 mg/dL), and reader concordance within these three groups was assessed.
We observed almost perfect agreement between readers for: 1) individual patients (yes/no) (n=50, 100% agreement, kappa=1.000); 2) total joints (n=200, 99% agreement, kappa=0.942); 3) FAC (n=100, 99% agreement, kappa=0.942); and 4) 1st MTPs (n=100, 99% agreement, kappa=0.942). Furthermore, findings by side (right/left) and diagnostic group (gout, AH, control) showed substantial to almost perfect concordance for all measures. MSU deposition was seen most commonly in gout patients, and deposition was also seen in some patients with asymptomatic hyperuricemia, but in only one control.
MSK-US is reliable for detecting MSU deposition in FAC and 1st MTPs in gout and AH.