Attending to the lightness of numbers: toward the understanding of critical care epidemiology.

Most of the epidemiological studies in critical care do not express their results in terms of population burden of critical illness. This happens because the population at risk of critical illness is particularly difficult to estimate, once intensive care units (ICUs) receive patients from many sources. The study by Laupland in this issue of Critical Care provides a good estimate of the incidence of admission to ICUs in the Calgary Health Region. He considered the Calgary Health Region population as the denominator and explored the effects of a changing numerator according to the residency status (resident in Calgary or not) on the estimation of the burden of admission to the ICU. He demonstrated that if the residency status were not known, the incidence of admission to the ICU would have been overestimated by more than 50%. Furthermore, non-residents had a lower mortality despite higher Acute Physiology and Chronic Health Evaluation (APACHE) II and Therapeutic Intervention Scoring System (TISS) scores. There is tremendous variability in decisions to admit a patient to the ICU and the epidemiology of critical care is influenced by them in a subtle but inextricable way. An understanding of the population epidemiology of critical illness and the use of the ICU, the variations in these parameters, and factors that influence this variation is extremely important. The notable effect of a changing numerator on the estimation of the population burden of ICU admissions in the study by Laupland illustrates how fluid our estimates of disease incidence and mortality - the mainstays of epidemiology - can be.