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The collapsibility index of the inferior vena cava is traditionally visualized from the subcostal region in the sagittal plane, referred to here as cIVCSS. Alternatively, the collapsibility index of the inferior vena cava can be visualized from the right midaxillary line in the coronal plane, referred to here as cIVCRC. It is unclear whether values of cIVCRC are comparable with values of cIVCSS because the inferior vena cava collapses asymmetrically into an elliptical form, quantified as the flat ratio of the inferior vena cava (F-IVC). This study aimed (1) to establish if cIVCRC is concordant or discordant to cIVCSS, and (2) to describe how this concordance or discordance is related to F-IVC.This single-center cross-sectional study enrolled 110 spontaneously breathing patients. Values of cIVCRC were compared with cIVCSS. Performance of cIVCRC ≥ 42% in predicting fluid responsiveness, defined as cIVCSS ≥ 42%, was assessed. F-IVC was also correlated to the difference between cIVCSS and cIVCRC.cIVCRC ≥ 42% was 61.5% sensitive (95% CI, 31.58%-86.14%) and 67.1% specific (95% CI, 55.81%-77.06%) for predicting cIVCSS ≥ 42%. cIVCRC underestimated cIVCSS. The degree of discordance between cIVCRC and cIVCSS was proportional to the value of F-IVC.cIVCRC and cIVCSS measures are discordant, where cIVCRC underestimates cIVCSS. The degree of discordance is directly proportional to the value of F-IVC. Therefore, we recommend that cIVCRC ≥ 42% be used to rule in, but not to rule out, fluid responsivity. Wherever possible, F-IVC should be assessed to understand the clinical relevance of cIVCRC.