Calculating Nosocomial Pneumonia Rates

A rate is an expression of the occurrence of an event. The time period must be specified and be identical for the numerator and denominator for the rate to be meaningful. Three kinds of rates are used in nosocomial infection surveillance incidence, prevalence, and incidence density. 

Incidence is the number of new cases of disease that occur in a defined population during a specified period. The incidence of nosocomial infection 

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Prevalence is the total number of active (existing and new) cases of the disease in a defined population, either during a specified period (period prevalence) or at a specified point in time (point prevalence). The prevalence nosocomial infection rate is calculated simply by dividing the number of active nosocomial infections in patients surveyed by the number of patients surveyed. Because nosocomial pneumonias occur relatively infrequently, the period chosen for surveillance must be large enough for an adequate estimation of a hospital s prevalence rate and usually varies depending on the number of occupied beds in a hospital. In addition, these rates require risk adjustment, which is currently not available for interhospital comparison of prevalence rates. 

Incidence density is the instantaneous rate at which disease is occurring, relative to the size of the disease-free population. Incidence density is measured in units of the number of cases of disease per person per unit of time. An example of an incidence density that is commonly used in hospitals is the number of nosocomial pneumonias per 1000 ventilator-days. Incidence density is useful when the infection rate varies in a linear fashion to the length of time a patient is exposed to a risk factor (i.e., the longer the patient is exposed, the greater the chance of acquiring infection). For example  

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Three categories of data comprise the usual information collected on a patient with a nosocomial pneumonia demographic, clinical, and laboratory. Information describing important risk factors for infection should also be collected, but only if it will be analyzed and used by the hospital personnel. Corresponding denominator data should also be collected so that infection rates can be calculated. 

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