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Objective: Although electrocardiography (ECG) is recommended in all subjects with hypertension, no information is available on the influence exerted by random changes in the placement of electrodes on the day-to-day variability of ECG criteria for diagnosis of left ventricular hypertrophy (LVH)., Methods: In a multicentre, randomized study, two standard 12-lead ECG were recorded, 24 h apart, from 276 consecutive hypertensive patients (mean age 65 +/- 12 years, 49.6% men). Overall, 142 patients were randomized to ECG with the position of electrodes marked on the skin using a dermographic pen and 134 to traditional ECG without marking the position of electrodes. Day-to-day variability of ECG criteria for LVH was compared between the two groups., Results: Coefficients of variation (SD of the difference between paired voltage measurements divided by the mean value) varied consistently among subjects randomized to ECG without dermographic pen, ranging from 30% (R wave in lead I) to 81% (R wave in lead V5). Dermographic pen led to a lesser variability of ECG voltages with consequent reduction in the coefficients of variation, which ranged from 26% (R-wave amplitude in lead I) to 43% (R-wave amplitude in lead V5). The proportion of subjects who changed classification status for LVH ('reclassification rate') from the first to the second ECG session (LVH present in session 1 and absent in session 2, or vice versa) decreased for effect of dermographic pen from 11 to 4% (P = 0.040) with the Cornell voltage, from 19 to 11% (P = 0.029) with the Sokolow-Lyon voltage, and from 18 to 7% with the Romhilt-Estes criterion (P = 0.018), but not with other criteria. In particular, the typical strain and the Cornell strain were associated with the lowest reclassification rates regardless of dermographic pen., Conclusions: Random changes in the position of ECG electrodes strongly impair the day-to-day reproducibility of Cornell voltage, Sokolow-Lyon and Romhilt-Estes criteria for LVH. The typical strain and Cornell strain criteria showed a lesser spontaneous day-to-day variability.