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Inter- and Intra-observer agreement in Measuring Urolithiasis Density on Non-Enhanced Computed Tomography.

INTRODUCTION
Treatment choice for urolithiasis is partially based on measuring stone density in Hounsfield Units (HU) on Non-Enhanced Computed Tomography (NECT). Inter-observer variability in these measurements could have treatment consequences. This study aims to assess the observer agreement of measuring HU and whether the use of a protocol leads to a better agreement.


MATERIALS AND METHODS
We retrospectively included 155 consecutive NECTs of patients with stones ≥4 mm. Five observers (two radiologists, one urologist, one urology resident, and one radiology resident) assessed all anonymized NECTs four times in randomized order. HU was measured without instruction (rounds 1;2) and subsequently using two protocols (A;B, rounds 3;4). Protocols comprised of using bone setting, using zoom, and measuring HU without the penumbra, in either three (A) or one (B) axial plane. The inter- and intra-observer agreement were evaluated using the Intraclass Correlation Coefficient (ICC).


RESULTS
Inter-observer agreement on HU measurement without protocol was: ICC=0.84 (CI-0.79-0.87). Agreement diminished with protocol A to ICC=0.62 (CI-0.37-0.76) and improved with protocol B to ICC=0.90 (CI-0.86-0.92). Intra-observer agreement without protocol was ICC=0.87, with protocol A ICC=0.87, and with protocol B ICC=0.93. The biggest improvement was seen for urologists' agreement from no protocol to protocol B, where ICC improved from 0.81 (CI-0.70-0.87) to 0.91 (CI-0.84-0.94).


CONCLUSIONS
Observer agreement of HU measurement of urolithiasis without protocol is already good but using zoom, bone setting, and measuring in a representative plane is recommended. This protocol results in higher agreement, especially among urologists. Measuring in 3 axial planes does not increase agreement.