Effect of number of replicate electrocardiograms recorded at each time point in a thorough QT study on sample size and study cost

J Clin Pharmacol. 2011 Jun;51(6):908-14. doi: 10.1177/0091270010376962. Epub 2010 Sep 17.


In a "thorough QT/QTc" (TQT) study, several replicate electrocardiograms (ECGs) are recorded at each time point to reduce within-subject variability. This decreases the sample size but increases the cost of ECG analysis. To determine the most cost-effective number of ECG replicates, the authors retrospectively analyzed data from the placebo and moxifloxacin arms of a TQT study with crossover design. Six replicate ECGs were recorded at 7 time points on day -1 (baseline day), day 1, and day 3 in 124 normal healthy volunteers who were randomized to receive moxifloxacin or placebo on day 1 and the other treatment on day 3. QT interval was corrected for heart rate by the Fridericia (QTcF) and individual subject-specific (QTcI) formulas. Within-subject and between-subject standard deviations for QTcF obtained by repeated-measures analysis of covariance were 9.5 and 13.3 milliseconds with 1 replicate; 7.8 and 12.7 milliseconds with 2 replicates; 7.3 and 12.3 milliseconds with 3 replicates; 6.9 and 12.2 milliseconds with 4 replicates; 6.8 and 11.9 milliseconds with 5 replicates; and 6.6 and 11.8 milliseconds with 6 replicates. Within- and between-subject variance with QTcI also declined with increasing replicates. Sample size benefit based on these estimates was negligible beyond 4 replicates. The study cost was least with 3 or 4 replicates, depending on per-ECG and per-subject costs.

Publication types

  • Randomized Controlled Trial

MeSH terms

  • Adult
  • Aza Compounds / adverse effects*
  • Cost-Benefit Analysis
  • Electrocardiography / drug effects
  • Electrocardiography / economics*
  • Electrocardiography / statistics & numerical data*
  • Female
  • Fluoroquinolones
  • Humans
  • Male
  • Middle Aged
  • Moxifloxacin
  • Quinolines / adverse effects*
  • Sample Size
  • Toxicity Tests, Acute / economics*
  • Toxicity Tests, Acute / methods


  • Aza Compounds
  • Fluoroquinolones
  • Quinolines
  • Moxifloxacin