Thursday, July 16, 2009

Question 4

A new test is developed that detects almost all cases of disease X within a population before the emergence of symptoms in an individual. Although the disease is detected early, there is no cure for the disease. However, there are medications available for decreasing potential complications arising out of the disease, thus prolonging life. The new test developed would lead to:

A) Decreased sensitivity and increased prevalence of the disease X
B) Decreased sensitivity and decreased prevalence of the disease X
C) Increased sensitivity and increased prevalence of the disease X
D) Increased sensitivity and decreased prevalence of the disease X
E) Decreased specificity and increased prevalence of the disease X
F) Increased specificity and increased prevalence of the disease X
G) Increased specificity and increased Positive predictive value
F) Increased sensitivity and increased Positive predictive value

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1 comments:

Usmle Challenger July 16, 2009 at 11:04 PM  

Answer: C) Increased Sensitivity and Increased Prevalence of the disease X

Points to note:

Sensitivity: Rules out disease (Identifies True Positives)
The test has highest sensitivity if it includes all those who are diseased.
Sn = TP/(TP+FN)

Specificity: Rules in (Identifies True Negatives)
The test has highest specificity if it includes all those who are not diseased (Well people).
Sp = TN/(TN+FP)

Prevalence = Incidence x Duration
P is the total number of ill cases over a given at period/point of time in a given population
Incidence is the number of new cases over a time duration.
So if life span of total number of ill patients is increased, P will increase. P is higher for chronic illness. A classic example is the total number of HIV patients have increased after the advent of Antiretroviral Treatment.

Predictive values deals with probability of a disease. So if any question asks about "CHANCE" of a test being a true positive or a true negative, think of Prevalence. If prevalence of disease is high (Chronic illnesses: DM/HTN), then PPV is high and NPV is low, and vice versa.

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