COMMUNITY MEDICINE~ MEASURES OF FREQUENCY OF DISEASES~

DISCUSSION

1) Epidemiologic measures of disease frequency.

2) Measurement of disease incidence and prevalence.

EPIDEMIOLOGIC MEASURES OF DISEASE FREQUENCY

Count:• Number of individuals who meet the

definition of the case.• Simple and useful• Example:

9188 cases of colorectal cancer in Pennsylvania in 2005.

Proportion:• A fraction of (A/A+B)• A= numerator:

Includes only number of indivuals who meet the case definition.• A+B= denominator:

Total numbers of individuals meet the case definition + those in the study population who do not meet the case definition and are at risk.

• Not dependant upon time.• May be present in fraction or percentage.• Linked to estimating risk.• Example:

30% of persons over the age of 50 have been screen for colon cancer.

Ratio:• A:B• A= nomerator

Includes only number os individuals meeting the case definition.

• B= denominator Includes only number of

individuals do not meet case definition and are at risk.

• Not dependant upon time.• Used infrequently.• Example:

2:1 case of depression in female and male.

Rate:• Nomerator is disease

frequency.• Denominator is the

population size.• Dependant upon time.• Example:

44 cases of colon cancer per 100,000 population in Pennsylvania during 2000.

Risk:• Probability of an

individual meeting the case definition.

• Dependent upon time.• Example:

0.00044 cases of colon cancer per person in a year.

MEASUREMENT OF DISEASE INCIDENCE AND PREVALENCE

Incidence: Measures new cases of disease develop over period of time. Identify risk factors. Estimated from clinical trials. Assess aetiology.

Prevalence: Measures both new and existing cases of disease at a

particular time or over period of time. Does not consider person die before the study begins. Estimated from cross-sectional studies and case-control

studies. Useful in planning of health services.

• To illustrate how incidence and prevalence differ, we consider our experience with AIDS.

• The number of annual incident cases of AIDS in gay men decreased in the US from the mid-1980's to the late 1990's. This has resulted primarily both from recent anti-retroviral treatment approaches and from prevention strategies for reducing high risk sexual behaviour.

• In contrast, the annual prevalent cases of AIDS in gay men has greatly increased in the US during the same period because recent treatment approaches for AIDS have been successful in prolonging life of persons with the HIV virus and/or AIDS.