Objective To establish a general method to estimate the measuring error in QT dispersion (QTD) determination, and to assess this error using a computer program for automated measurement of QTD.

Subjects Measurements were done on 1220 standard simultaneous 12 lead electrocardiograms.

Design The computer program was validated against two observers on a random subset of 100 electrocardiograms. Simple laws of physics require that at least five of the six extremity leads have the same QT duration. This allows the direct assessment of the error in measuring QTD derived from five extremity leads (QTD₅). It also enables ST-T amplitude dependent distributions of measurement error in determining QT duration to be established. These QT error distributions were then used to estimate the error in measuring QTD from all 12 leads (QTD₁₂).

Main outcome measures Mean and standard deviation of error in measuring QT duration, QTD₅, and QTD₁₂.

Results Performance of the program was comparable to that of observers. Errors in measuring QT duration (measured QT minus reference QT) fell from a mean (SD) of 6.9 (17.1) ms for ST-T amplitudes < 50 μV to −1.4 (6.3) ms for amplitudes > 350 μV. Measurement errors of QTD₅ and QTD₁₂ were 20.4 (11.5) ms and 29.4 (14.9) ms.

Conclusions The fact that no QTD can exist between five of the six extremity leads provides a means of estimating QTD measurement error. Measuring error of QT duration is dependent on ST-T amplitude. QTD measurement error is large compared with typical QTD values reported.