title: Behavior of cross-sectional surveys in the hospital setting: a simulation model.
abstract: OBJECTIVE
  To date, it has not been adequately proven whether the published formulas used to obtain incidence from the prevalence of nosocomial infections provide a good estimate of real incidence. With the hypothesis that within the hospital setting prevalence may be lower than incidence, the aim of this study was to analyze the behavior of point prevalence as it relates to cumulative incidence and duration of infection.
  
  
  METHODS
  Hospital simulation study.
  
  
  METHODS
  By randomly selecting a sample of infected patients within a specific range of cumulative incidences and infection durations, we constructed a simulated hospital population, allowing us to estimate daily point prevalences and their maximum and minimum values. The association between the different components of stay and cumulative incidence was evaluated to obtain a more accurate estimate of incidence.
  
  
  RESULTS
  Prevalence can be lower than, equal to, or higher than the corresponding incidence. For all incidence levels, prevalence was increasing with duration. Between 14 and 20 days of infection duration, prevalence was consistently lower than incidence. Prevalence duration of infection was approximately half the time of the total duration.
  
  
  CONCLUSIONS
  The existing formulas relating incidence and prevalence can frequently be inadequate. Until a validated system for converting prevalence into incidence is available, we do not believe their use is appropriate.
authors.affiliation: Servicio de Medicina Preventiva y Epidemiologia, Hospital Universitario Vall d'Hebron, p Vall d'Hebron 119-129, 08035 Barcelona, Spain. jrossello@vhebron.net
authors.forename: Jose
authors.initials: J
authors.lastname: Rossello-Urgell
authors.forename: Alicia
authors.initials: A
authors.lastname: Rodriguez-Pla
mesh_terms: D016001:Confidence Intervals; D003428:Cross Infection / Q000453:epidemiology*; D003430:Cross-Sectional Studies; D006801:Humans; D015994:Incidence; D007902:Length of Stay; D016014:Linear Models