Thanks for all the answer for the question Calculation of incidence rate for epidemiological study in hospital. And here come's the second part of the question:
What about the prevalence rate then? I have read The new public health as suggested by Chi, the book says that prevalence is usually not available when using ordinary incidence rate, but I saw another formula here:
total case count in that period of time/total patient bed days during that period of time
It puzzled me again, what is it? I have never heard of prevalence calculated using denominator as patient-bed days.
Thanks!
Best Answer
It's a somewhat unusual way to calculate prevalence, but it makes some sense to use the patient-bed days as the denominator. Consider two scenarios:
A hospital has a single patient, who stay for 1,000 days, and in that time, has a single infection.
B hospital has 1000 patients, who stay for 1 day each, and in that time, they have 50 infections.
Using "N" as the denominator:
A Prevalence = 1.00 B Prevalence = 0.05
Using Patient-Days as the denominator:
A Prevalence = 0.001 B Prevalence = 0.05
The latter accurately reflects the higher burden of disease. Generally speaking, when you calculate a prevalence just using N, it's under the assumption that all persons at risk are at risk for the same amount of time. In the example above, that isn't true.