Case-Fatality Rate

Case-fatality rate (CFR) is a clinical measure that describes a person's likelihood of dying from a disease [Page 134]once he or she is diagnosed with it. It is calculated by dividing the number of deaths from a specified disease during a specified time by the number of individuals who have that disease during that time and multiplying this ratio by 100. It is a measure of disease severity and is often used for prognosis. It can also be used to evaluate the effect of new treatments—with improved treatments, the CFR is expected to decrease. The CFR is not a constant; it can vary between populations and over time depending on the interplay between agent, host, and environment, as well as available treatments and quality of care. The higher the CFR, the higher the likelihood of dying from the disease.

Although CFR has a simple formula, getting an accurate estimate of CFR is not simple. One of the difficulties in estimating CFR is ensuring the accuracy of the numerator. This becomes harder as duration of the disease of interest lengthens because a person becomes more likely to die of another cause prior to death from the disease itself. As a result, the CFR may be underestimated because people who die from another cause will not be counted in the numerator although they may have died from the disease at a later date had they not succumbed to something else first. For an accurate estimate of CFR, one also must be certain that those included in the numerator actually died from the disease in question. If this number includes people who died from other causes, the CFR will be overestimated. These difficulties explain why CFR tends to be a measure used for acute infectious diseases or diseases with short duration rather than for chronic diseases or diseases with long duration.

The denominator used to calculate CFR can also pose a challenge for an accurate estimate. If less severe cases are missed, and therefore not included in the denominator, CFR will be overestimated. For example, underestimating CFR may occur because of an inaccurate denominator that is determined during the early stages of an investigation when people who have the disease in question are missed because they died of the disease prior to the investigation starting.

It is important to point out the difference between CFR and mortality rate. Although number of deaths is the numerator for both, mortality rate is calculated by dividing the number of deaths by the population at risk during a certain time frame, and as a true rate, it estimates the risk of dying of a certain disease. As an example, let us consider two populations. One of them has 1,000 people; of these, 300 people have the disease and 100 people die of it. In this case, the mortality rate for the disease is 100/1,000 = 0.1, or 10%, and the CFR is 100/300 = 0.33, or 33%. The other population also has 1,000 people but 50 people have the disease and 40 die from it. Here, the mortality rate is 40/1,000 = 0.04, or 4%, and the CFR is 40/50 = 0.8, or 80%. The incidence of death from the disease is higher in the first population, but the severity is greater in the second. This points to the fact that the two measures provide us with different information.

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