Re: Cardiovascular outcomes associated with use of clarithromycin: population based study
Thank you for the response of Dr. Blake. We have read through your comments and we hope the following response will address the issues you raised about our study.
In our paper, we attempted to use three study designs to conduct studies with two cohorts of patients to investigate the association between clarithromycin and cardiovascular outcomes. The first was a propensity score adjusted cohort design comparing exposure to clarithromycin with exposure to amoxicillin/ amoxicillin with clavulanate potassium. The second was a self-controlled case series design comparing the rate of cardiovascular events after exposure to Helicobacter pylori therapy containing-clarithromycin with baseline of follow up. The third study design was a case-crossover study design comparing odds of exposure to H. pylori therapy containing-clarithromycin in 14 days before a cardiovascular event with the odds of exposure during control periods. The latter two designs are both case-only designs using the same cohort, but the designs are considerably different.1-2
Dr. Blake raises an important concern regarding the self-controlled study designs. An indication for testing and treating for H. pylori infection is epigastric pain. If angina were misdiagnosed as epigastric pain due to gastritis, this could create a spurious temporal association between treatment for H. pylori infection and myocardial infarction. While this non-causal mechanism cannot be entirely ruled out the fact that an elevated risk was only seen for the duration of treatment would be unusual for this mechanism. In addition, the elevated short term risk of arrhythmia is congruent with our hypothesis that the risk is due to the clarithromycin prolonging the QT interval. Our study findings also supported the current literature that there was an increased short-term risk of cardiac death including myocardial infarction and arrhythmia associated with clarithromycin.3
Concerning the cardiovascular safety of proton pump inhibitors, a recent meta-analysis suggested that patients receiving both clopidogrel and proton pump inhibitors may be at higher cardiovascular risk but that this did not represent a direct cause of ischaemic events.4 Other studies also showed no evidence of an increased cardiovascular events including myocardial infarction after adjustments among patients using proton pump inhibitors.5-8 Therefore there is some evidence that PPIs are a marker of ill health rather than a direct cause of events such as myocardial infarction. Given the conflicting results in recent studies, it is warranted to conduct future studies to validate the findings of the association between the risk of myocardial infarction and proton pump inhibitors.
We chose amoxicillin/amoxicillin with clavulanate potassium as the comparator as it has not been reported to be associated with cardiovascular events and it has similar indications to clarithromycin. Choosing a comparator with similar indications serves as a proxy for selecting groups of patients with similar baseline characteristics. With additional use of propensity score modelling, comparison was made between two groups of patients with similar characteristics.9 Many other observational studies investigating the association between cardiovascular events and macrolides also chose amoxicillin/ amoxicillin with clavulanate potassium as comparator.10-12 We are aware that doxycycline is also given for patients with community acquired pneumonia in Hong Kong. However, recent literature has explored and seems to support its cardio-protective effect, and so we decided it would not have been an ideal comparator in this context.13-14
With regard to the implications of our findings on current practice, we feel that it is prudent to exercise caution in prescribing clarithromycin to patients at high risk of cardiovascular events and especially in those with a history of cardiac arrhythmia. Our estimate of the risk of cardiovascular events during current use of clarithromycin needs to be balanced against the often large benefits of treatment. Whether the risks identified by our study should be discussed with the patient will ultimately be a clinical decision taken on a case by case basis and will depend on the absolute risk of harm from treatment, the availability of suitable alternatives and the risk of non-treatment. It should be noted that our recommendations are broadly in line with the Physicians' Desk Reference (PDR), British National Formulary and summary of product characteristics of clarithromycin.15-17
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Competing interests: No competing interests