title: Intra- and interreader variability in QT interval measurement by tangent and threshold methods in a central electrocardiogram laboratory.
abstract: BACKGROUND
  The QT interval can be measured by tangent (QT(Tan)) and threshold (QT(Thr)) methods; the better method is the one with lower reader variability.
  
  
  METHODS
  QT(Tan) and QT(Thr) were measured twice in 100 digital electrocardiograms (ECGs) by 8 experienced readers in a central laboratory. For QT(Thr), the end of the T wave was the point where the T wave reached the isoelectric baseline; for QT(Tan), 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.
  
  
  RESULTS
  The average absolute intrareader variability ranged from 3.4 to 6.9 milliseconds for QT(Tan) and from 3.5 to 5.2 milliseconds for QT(Thr). By analysis of variance, intrareader SD of QT(Tan) was 7.0 and 7.5 milliseconds for QT(Thr); interreader SD was 13.1 milliseconds for QT(Tan) and 11.9 milliseconds for QT(Thr). QT(Tan) was shorter than QT(Thr) in 96 of the 100 ECGs, it exceeded QT(Thr) in 4 ECGs, which had prominent U waves.
  
  
  CONCLUSIONS
  For 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 QT(Tan) and QT(Thr). However, QT(Tan) is consistently shorter than QT(Thr) by up to 10 milliseconds.
authors.affiliation: Research Section, Quintiles ECG Services, Mumbai, India. gopi.panicker@quintiles.com
authors.forename: Gopi Krishna
authors.initials: GK
authors.lastname: Panicker
authors.forename: Dilip R
authors.initials: DR
authors.lastname: Karnad
authors.forename: Mili
authors.initials: M
authors.lastname: Natekar
authors.forename: Snehal
authors.initials: S
authors.lastname: Kothari
authors.forename: Dhiraj
authors.initials: D
authors.lastname: Narula
authors.forename: Yash
authors.initials: Y
authors.lastname: Lokhandwala
mesh_terms: D000465:Algorithms; D001145:Arrhythmias, Cardiac / Q000175:diagnosis*; D003936:Diagnosis, Computer-Assisted / Q000379:methods*; D004562:Electrocardiography / Q000379:methods*; D006801:Humans; D015588:Observer Variation; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity