Background: Monitoring of the QT duration by electrocardiography (ECG) prior to treatment is frequently recommended in the label of QT prolonging drugs. It is, however, unknown how often these risk minimisation measures are being adhered to by general practitioners in daily clinical practice. Objectives: We assessed the frequency of ECG measurements in patients where haloperidol was initiated in primary care. Methods: A prospective cohort study was performed in the UK Clinical Practice Research Datalink. Patients aged 18 years or older with a first prescription of haloperidol between January 1, 2009 and May 1, 2013 were included. The proportion of ECGsmade was determined in two blocks of four weeks; during the exposure period when haloperidol was initiated, and during the control period, one year before. Conditional logistic regression analysis was applied to calculate the relative risk of having an ECG in the exposure period compared to the control period. Subgroup analyses were performed to assess the proportion of ECG measurements in patients with one or more additional risk factors for QT prolongation. Results: In total, 3420 patients were prescribed haloperidol during the exposure period, and 1.8% of them had an ECG at treatment initiation, compared to 0.8% during the control period (relative risk [RR] 2.4 [1.5-3.8]). Of the patients with additional risk factors for QT prolongation 1.9% of the patients had an ECG at initiation of the prescription, compared to 1.0% during the control period (RR 2.1 [1.2-3.5]). Conclusions: Compliance with recommendations to perform an electrocardiogram when starting a new QT prolonging drug is extremely low, when haloperidol is taken as an example
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