Preventive And Social Medicine

Submitted by: India Qbank

Q. In a community, an increase in new cases denotes:

A. Increase in incidence rate

B. Increase in prevalence rate

C. Decrease in incidence rate

D. Decrease in prevalence rate

Ans. A

Explanation:

Incidence refers to new cases of disease occurring among previously unaffected individuals. This is a more appropriate measure for etiological studies of cancer and other chronic illnesses, wherein one attempts to relate disease occurrence to genetic and environmental factors in a framework of causation. The duration of survival of

patients with a given disease, and hence its prevalence, may be influenced by treatment and other factors which come into play after onset.

Rates, as opposed to frequencies, imply an element of time. The rate of occurrence of an event in a population is the number of events which occur during a specified time interval, divided by the total amount of observation time accumulated during that interval. For an incidence rate, the events are new cases of disease occurring among

disease-free individuals. The denominator of the rate can be calculated by summing up the length of time during the specified interval that each member of the population was alive and under observation, without having developed the disease. It is usually expressed as the number of person-years of observation. Mortality rates, of course,

efer to deaths occurring among those who remain alive.

Q. The same screening test is applied to two communities X and Y; Y shows more false +ve cases as compared to X. The possibility is:

A. High sensitivity

B. High specificity

C. Y community has high prevalence

D. Y community has low prevalence

Ans. C

Explanation:

The probability of having the disease, given the results of a test, is called the predictive value of the test. Positive predictive value is the probability that a patient with a positive (abnormal) test result actually has the disease. Negative predictive value is the probability that a person with a negative (normal) test result is truly free of disease. Predictive value is an answer to the question: If my patient’s test result is positive, what are the chances that my patient does have the disease?

Predictive value is determined by the sensitivity and specificity of the test and the prevalence of disease in the population being tested. (Prevalence is defined as the proportion of persons in a defined population at a given point in time with the condition in question.) The more sensitive a test, the less likely an individual with a negative test will have the disease and thus the greater the negative predictive value. The more specific the test, the less likely an individual with a positive test will be free from disease and the greater the positive predictive value.

When the prevalence of preclinical disease is low, the positive predictive value will also be low, even using a test with high sensitivity and specificity. For such rare diseases, a large proportion of those with positive screening tests will inevitably be found not to have the disease upon further diagnostic testing. To increase the positive predictive value of a screening test, a program could target the screening test to those at high risk of developing the disease, based on considerations such as demographic factors, medical history or occupation. For example, mammograms are recommended for women over the age of forty, because that is a population with a higher prevalence of breast cancer.

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