Background/Purpose: To evaluate limits of detection of ultrasound to detect monosodium urate crystals suspended in synovial fluid.

Methods: Serial dilutions were made mixing liquid tophus aspirated from an olecranon bursa with crystal free synovial fluid aspirated from a patient with knee osteoarthritis. For each dilution, beginning with the undiluted liquid tophus aspirate, crystal free synovial fluid was mixed with each subsequent dilution in a 1:1 ratio creating dilutions from 1:1 to 1:1024. At each dilution, the MSU suspensions in eppendorf tubes were scanned with GE Logic linear probe settings ranging from low to high frequency as needed for optimal imaging and penetration. Compensated polarized light microscopy images at 10x and 40x were obtained for each dilution.

Results: Ultrasound evidence of MSU crystals (snowstorm appearance) were still evident in serial dilutions up until 1:128. Corresponding histology imaged by compensated light microscopy revealed that large crystal aggregates were rare but there were still a high number of crystals (~100) per low power field. Further dilutions resulted in ultrasound images that were indistinguishable from crystal free aspirates of synovial fluid despite persistence of lower frequency MSU crystals.

Conclusion: Ultrasound is sensitive for detecting modest frequency of MSU crystals suspended in synovial fluid. Never-the-less, MSU crystals are still present in synovial fluid beyond ultrasound detection.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/limits-of-detection-of-monosodium-urate-crystals-in-synovial-fluid-by-ultrasound/