External caliper-based measurements of the modified percent depth as an alternative to cross-sectional imaging for assessing the severity of pectus excavatum

Background: Cross-sectional imaging (CSI) may be clinically unnecessary in the evaluation of pectus excavatum (PE). The purpose of our study was to prospectively evaluate the accuracy and reliability of the modified percent depth (MPD), derived from caliper-based external measurements, in identifying PE. Methods: Children 11–21 years old presenting for evaluation of PE or to obtain thoracic cross-sectional imaging for other indications were measured to derive the Modified Percent Depth. The Haller Index (HI) and Correction Index (CI) were calculated from CSI. Receiver–Operator Characteristic (ROC) analysis was used to compare the sensitivity and specificity of MPD, HI, and CI. Interrater reliability was assessed using Spearman's correlation coefficient and Cohen's Kappa coefficient. Results: Of 199 patients, 76 (38%) had severe PE. Median age was 16 years (range = 11–21). The median Modified Percent Depth was 21.4% (IQR = 16.2–26.3) among those with PE versus 4.1% (IQR = 1.7–6.4) in those without (p < 0.001). MPD ≥ 11% exhibited similar sensitivity and specificity to HI ≥ 3.25 and CI ≥ 10 for identifying PE (ROC 0.98 vs. 0.97 vs. 0.98, respectively, p = 0.41). With respect to interrater reliability, independent clinicians' caliper measurements exhibited 87% agreement when identifying MPD ≥ 11% (p < 0.001) with excellent correlation (Spearman's ρ > 0.71, p < 0.001). Conclusion: Caliper-based, physical examination measurements of the Modified Percent Depth reliably identify pectus excavatum and represent an alternative to CSI-based measurements for the assessment of PE. Type of study: Diagnostic test. Level of evidence: Level II.