Reproducibility of musculoskeletal ultrasound for determining monosodium urate deposition: concordance between readers.

OBJECTIVE: Criteria for sonographic diagnosis of monosodium urate (MSU) crystal deposition have been developed, but the interreader reproducibility of this modality is not well established. We therefore assessed agreement using a systematic approach.
METHODS: Fifty male subjects ages 55-85 years were recruited during primary care visits to an urban Veterans Affairs hospital, and were assessed by musculoskeletal ultrasound (US) of the knees and first metatarsophalangeal (MTP) joints to evaluate for the double contour sign and tophi as evidence of MSU crystal deposition. Images were read by 2 blinded rheumatologists trained in musculoskeletal US, and the degree of concordance was determined for individual subjects, total joints, femoral articular cartilage (FAC), and first MTP joints. Subjects were further categorized into 3 diagnostic groups: gout, asymptomatic hyperuricemia (no gout, serum uric acid [UA] ≥6.9 mg/dl), and controls (no gout, serum UA ≤6.8 mg/dl), and reader concordance within these 3 groups was assessed.
RESULTS: We observed almost perfect agreement between readers for 1) individual subjects (yes/no; n = 50, 100% agreement, κ = 1.000), 2) total joints (n = 200, 99% agreement, κ = 0.942), 3) FAC (n = 100, 99% agreement, κ = 0.942), and 4) first MTP joints (n = 100, 99% agreement, κ = 0.942). Furthermore, findings by side (right/left) and diagnostic group (gout, asymptomatic hyperuricemia, 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 subjects with asymptomatic hyperuricemia, but in only 1 control.
CONCLUSION: Musculoskeletal US is reliable for detecting MSU deposition in FAC and first MTP joints in gout and asymptomatic hyperuricemia.