Situation: The public health nurse of a Rural Health Unit (RHU) analyzes health data for the annual community diagnosis of a municipality with a population of 45,000, of whom 18,000 are adults aged 20 years and older. During this year's rainy season, 250 persons were reported as dengue cases, and 7 of them died. Laboratory testing later ruled out dengue in 50 of the reported persons, one of whom was among the 7 who died (of another illness). The remaining 200 reported persons met the case definition of confirmed or probable dengue. The other 6 deaths were due to dengue in confirmed or probable cases. Counting only confirmed and probable cases, what is the case fatality rate of dengue? Round off to one decimal place.

Both the numerator and the denominator must follow the case definition. Dengue cases = 250 − 50 ruled out = 200; dengue deaths = 7 − 1 death in a ruled-out person = 6. Case fatality rate = 6 ÷ 200 × 100 = 3.0%.

The case fatality rate (CFR) is a core measure of disease severity in community health nursing. It answers a specific question: Among persons who already have the disease, what proportion die from it? Unlike a crude death rate or an incidence rate, the CFR uses only confirmed and probable cases as its denominator, and only deaths attributable to that disease as its numerator .

In this municipality, the initial report of 250 dengue cases was reduced by 50 after laboratory testing ruled out dengue. This left 200 persons who met the case definition of confirmed or probable dengue. The same principle applies to the deaths. Of the 7 reported deaths, 1 occurred in a person whose dengue was ruled out, meaning that death was due to another illness. Therefore, only 6 deaths are counted as dengue deaths among confirmed or probable cases.

The calculation is:

CFR = 6 ÷ 200 × 100 = 3.0%

This result reflects the importance of aligning both numerator and denominator with the same case definition. If the 50 ruled-out persons had been left in the denominator, the CFR would be incorrectly low. If the 1 non-dengue death had been left in the numerator, the CFR would be incorrectly high. Watch out! The population of 45,000 and the adult count of 18,000 are distractors; the CFR does not use the total population as its denominator.

In epidemiological surveillance, distinguishing probable cases from laboratory-confirmed cases is essential because dengue reporting systems often include both categories ^[2]. A probable case is typically defined by clinical and epidemiological criteria, while a confirmed case requires laboratory evidence such as NS1 antigen, IgM seroconversion, or RT-PCR. When a case is later ruled out, it must be removed from both the case count and the death count to avoid overestimating or underestimating the true burden of disease .

For nursing licensure examinations, the key distinction is between the CFR and the mortality rate. The mortality rate uses the entire at-risk population as the denominator, while the CFR uses only diagnosed cases. This is why a disease can have a high CFR but a low mortality rate if few people become infected, or a low CFR but a high mortality rate if infection is widespread. Key point! When a question asks for the case fatality rate, look for the number of deaths from the disease divided by the number of confirmed or probable cases of that disease, then multiply by 100.

In community diagnosis, the CFR helps the public health nurse prioritize interventions. A CFR of 3.0% indicates that for every 100 confirmed or probable dengue cases, 3 died. This information guides decisions about vector control intensity, referral pathways, and health education on early warning signs of severe dengue .