E-1 Incidence and Prevalence

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Epi & Biostats Essentials – Module 1 from MarylandCCProject.org on Vimeo.

E-1: Measures of Disease Frequency – Incidence and Prevalence

Prevalence – the number of people with a disease at one point in time.

• All cases existing in the population at a time point
• Prevalence provides a snapshot of a population at a point in time where you see who has the disease and who does not. It doesn’t tell you when the disease developed
• The concept of prevalence can be expressed either just a number (the number of cases of disease) or a ratio of all cases of the disease divided by the population at risk

Incidence – a measure of new cases that occur during a specified period time

• Incidence is the number of new cases of disease divided by the population without the disease at baseline

• Incidence Density – number of new cases of a disease per person-time at risk

• Incidence is sometimes not displayed as a ratio but instead expressed as a number of cases.  For example, one could say that there were 20,000 incident cases of lung cancer in 2003.

Comparing Incidence and prevalence

• Incidence gives an estimation of risk of contracting a disease
• Prevalence = Incidence X duration of disease (when there is a steady state situation without patient migration or change in disease rates)

Example  #1 – Severe Sepsis Survival

Population Burden of Long-Term Survivorship After Severe Sepsis in Older Americans

• This was a retrospective cohort analysis of medicare data examining patient survival and disability 3 years after severe sepsis.
• The incidence and prevalence of 3-year survivorship in severe sepsis were calculated and compared from 1999 to 2008 to assess changes occurring in the past decade.
• There were 387,330 cases of severe sepsis in 1996 of which 102,767 survived to 1999. In 2005 there were 791,809 cases of severe sepsis of which 225,251 survived to 2008.
• In 2008, there were 637,867 patients who had survived severe sepsis by at least 3 years. There were 39,337,348 patients examined.
• What were the incidence and incidence rate of 3-year survival in 1999? In 2008?
• What was the prevalence of survival in 2008?

Example #2 Ventilator-associated Pneumonia

Silver-coated endotracheal tubes and incidence of ventilator-associated pneumonia: the NASCENT randomized trial

• A silver-coated endotracheal tube was designed to reduce ventilator-associated pneumonia (VAP) incidence by preventing bacterial colonization and biofilm formation
• This was a prospective, randomized study where patients were intubated with similar ETT except for silver coating
• Of 766 patients in the silver-coated ETT arm, there were 37 cases of VAP. Of the 743 patients in the control arm of the trial, there were 56 cases of VAP.
• What was the incidence rate of VAP in the two groups?

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