Comorbidity

A modified Charlson Comorbidity Index was used to adjust for confounding by co-morbidities [19, 20]. The index contains seven diagnoses with the weight of one (myocardial infarction, congestive heart failure, cerebrovascular disease, dementia, chronic pulmonary disease, connective tissue disorder and diabetes without complication) and two diagnoses with the weight of two (moderate or severe renal failure and any tumour), which gives a total of maximum eleven. The comorbidity index was entered as a categorical variable in the analysis (none, one, two and three or more modified Charlson index weights). All diagnoses except renal failure and dementia diagnosis were based on medical history and examination of the physician. Renal failure was calculated from Cockgroft-Gaults formula [21] and defined as creatinine clearance < 25 ml/min. The dementia diagnosis procedure is described above.