To the Editor: Advocating implementation of evidence-based clinical practice guidelines is one aspect of the current drive to provide quality healthcare across different centres.

Quality theory demands that outcomes are continuously sought and that practices are modified accordingly — the "quality loop".

Therefore, Scott and Harper are to be applauded for their pursuit of improved outcomes, not just improved processes, in studying guideline-discordant care in acute myocardial infarction.1 I believe that this type of study, which objectively demonstrates the role of practice guidelines in "real world" practice, is very important.

However, as a geriatrician, my patient population is unlikely to intersect with populations enrolled in large cardiology trials (eg, those for thrombolysis in myocardial infarction).2,3 Comorbidities, such as renal impairment, cognitive impairment and poor functional status at baseline, were not explicit exclusion criteria, but, when present, would have reduced an individual's chance of being enrolled.

These types of comorbidities are likely to be associated with a reluctance on the part of patients and physicians to pursue life-prolonging interventions. They are also likely to be associated with poorer outcomes, whatever the intervention. Therefore, I believe that these non-cardiac comorbidities are potential confounders for study designs, such as that of Scott and Harper.1

Older age per se has been well studied in the cardiology literature on management of myocardial infarction. However, in the literature on adherence to guidelines, few studies have attempted to fully identify the non-cardiac-related characteristics of those receiving guideline-discordant care. Krumholz et al reported that altered mental state is one factor, and that, of a large "real-world" cohort aged 65 or more, only 8% were considered ideal candidates for thrombolytic therapy.4

Quality healthcare involves multiple dimensions, including both personal and process factors. Practice guidelines are valuable tools to reduce practice variation, but we need to continue to evaluate whether they can be applied as broadly as may be advocated.

Surely, evidence-based guidelines can only be confidently applied to situations for which an evidence base exists. It will be important to test the application of guidelines in many settings, with attention to potential confounders, and, in particular, to outcome measures.