Intra- and interreader variability in QT interval measurement by tangent and threshold methods in a central electrocardiogram laboratory

BackgroundThe QT interval can be measured by tangent (QTTan) and threshold (QTThr) methods; the better method is the one with lower reader variability.MethodsQTTan and QTThr were measured twice in 100 digital electrocardiograms (ECGs) by 8 experienced readers in a central laboratory. For QTThr, the end of the T wave was the point where the T wave reached the isoelectric baseline; for QTTan, it was the point where a line from the peak of the T wave through the steepest part of the descending limb intercepted the isoelectric baseline.ResultsThe average absolute intrareader variability ranged from 3.4 to 6.9 milliseconds for QTTan and from 3.5 to 5.2 milliseconds for QTThr. By analysis of variance, intrareader SD of QTTan was 7.0 and 7.5 milliseconds for QTThr; interreader SD was 13.1 milliseconds for QTTan and 11.9 milliseconds for QTThr. QTTan was shorter than QTThr in 96 of the 100 ECGs, it exceeded QTThr in 4 ECGs, which had prominent U waves.ConclusionsFor trained readers in a central ECG laboratory using sophisticated on-screen tools for QT measurement in high-quality digital ECGs, between- and within-reader variability are comparable for QTTan and QTThr. However, QTTan is consistently shorter than QTThr by up to 10 milliseconds.