https://www.infirmiere-canadienne.com/blogs/ic-contenu/2012/10/01/terminologie-101-incidence-cumulative-et-taux-dinc
Oct 01, 2012, By: Maher M. El-Masri, RN, PhD
Incidence: The number of new cases of a disease that occur during a specified period of time in a population at risk for developing the disease.
Source: Gordis, L. (2009). Epidemiology. (4th ed.). Philadelphia: Saunders Elsevier
Incidence provides information about the spread of disease. Cumulative incidence (CI) and incidence rate (IR) are different approaches to calculating incidence, based on the nature of followup time.
Let’s say that health-care professionals working in an intensive care unit have asked whether there has been an increase in the number of new pneumonia cases. First, researchers focus on the CI, which is calculated by dividing the number of new cases of a disease in a population during a specific period of time (the numerator) by the total number of people at risk of developing the disease in that population during the same period of time (the denominator). Three points apply: pre-existing cases of the disease cannot be included in the numerator; people who already have the disease or who are incapable of having the disease are excluded from the denominator; and the calculation is based on the presumption that all at-risk individuals are observed for an equal period of time.
The month of January is chosen as the study period. A total of 220 patients passed through the ICU during the month; of these, 20 were admitted with a diagnosis of pneumonia and 10 developed pneumonia during their stay. The 20 patients admitted with pneumonia are excluded from the denominator because they were not at risk, which yields 200 at risk. The numerator consists of the 10 at-risk patients who developed pneumonia. The CI of pneumonia is therefore 10/200 or five per cent in the one-month period. In other words, there is a risk that five per cent of new patients in the ICU will develop pneumonia in a one-month period.
Researchers next calculate the IR (also known as incidence density), which reflects variation in the lengths of time that at-risk individuals are observed. IR is especially useful in hospital surveillance, where patients may enter and leave the surveillance at varying times (e.g., patients may be lost to follow up or die from a cause other than that being studied). It is calculated by dividing the number of new cases of a disease by the total of the lengths of time that each individual in the population was at risk, expressed as person-time (e.g., person-days). In our example, let us assume that the 200 at-risk patients were in the ICU for a total of 1,000 person-days during January. Since 10 of the 200 patients developed pneumonia, we conclude that the IR of pneumonia was 10 cases per 1,000 person-days (i.e., one case per 100 person-days). In other words, there is a risk that one new case will develop over 100 person-days.
Remember, CI tells us the risk of disease when the observation time is assumed to be equal for all patients in the denominator, while IR tells us the risk of disease when the observation time varies among individuals in the denominator.
NurseONE resources
MyiLibrary
- Maltby, J. (2010). Research methods for nursing and healthcare.
- Campbell, M. J., Machin, D., & Walters, S. J. (2010). Medical statistics: A textbook for the health sciences (4th ed.).
- Roberts, P., & Priest, H. (Eds.). (2010). Healthcare research: A textbook for students and practitioners.
- Waltz, C., Strickland, O., & Lenz, E. R. (2010). Measurement in nursing and health research (4th ed.).
Maher M. El-Masri, RN, PhD, is a full professor and research chair in the faculty of nursing, University of Windsor, in Windsor, Ont.
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