The Rule of 10: a simple 3D ultrasonographic method for the diagnosis of T-shaped uterus

Purpose To investigate and propose a new simple tridimensional (3D) ultrasonographic method to diagnose a T-shaped uterus (Class U1a). Methods A multicenter non-experimental case-control diagnostic accuracy study was conducted between January 2018 and December 2019, including 50 women (cases) diagnosed with T-shaped uterus (U1a class) and 50 women with a "normal uterus" (controls). All the enrolled women underwent 3D ultrasound, drawing four lines and recording the length of three of them as follow: draw and measure the interostial line (R0); draw from the midpoint of R0 a perpendicular line length 20 mm; draw and measure in the uterine cavity a line parallel to R0 at 10 mm below R0 (R10) and a second line parallel to R0 at 20 mm below R0 (R20). The diagnostic performance of all sonographic parameters statistically significantly different between T-shaped and normal uteri was estimated using the receiver operator characteristic (ROC) curve analysis. Results R10 and R20 were statistically significantly shorter in the T-shaped than the normal uterus. R10 reported the highest diagnostic accuracy with an area under the ROC curve of 0.973 (95% CI 0.940-1.000). R10 length maximizing the Youden's J statistic was 10.5 mm. Assuming R10 length equal to or shorter than 10 mm as the cut off value for defining a woman as having a T-shaped uterus, the new ultrasonographic method following the proposed protocol (R0, R10, and R20) reported sensitivity for T-shaped uterus of 91.1% (95% CI 0.78-0.97%) and a specificity of 100% (95% CI 0.89-100%). The positive likelihood ratio was higher than 30, and the negative likelihood ratio was 0.09 (95% CI 0.04-0.26). Conclusions Measuring the length of the intracavitary line parallel to the interostial line at 10 mm from it and using a length <= of 10 mm as cut off value (the "Rule of 10") appears a simple and accurate 3D ultrasonographic method for the diagnosis of a T-shaped uterus.