Incidence and Prevalence

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

E-1: Intro to Epi: Measures of Disease Frequency

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

1. All cases existing in the population at a time point
2. 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
3. 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

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

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

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

2003. 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
  1. Incidence gives an estimation of risk of contracting a disease
  2. 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?

Incidence of 3-year survival in 1999:

Incidence – 102,767

Incidence of 3-year survival in 2008:

Incidence – 225,251

Prevalence of survival in 2008:

Prevalence – 637,867

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?

Incidence of VAP in silver-coated arm

Incidence of VAP in control arm

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