In January, 2001 forty heterosexual hemophiliac patients (all males) were asked
ID: 3042723 • Letter: I
Question
In January, 2001 forty heterosexual hemophiliac patients (all males) were asked to participate in a 3 year prospective study . The men were to undergo an interview, physical examination, and blood testing for HIV status every 6 months for 3 years. Among the 40 subjects there were 30 who were seronegative and healthy for the entire duration of the study, and all of these were followed for the entire 3 years. During the initial screening, 2 of the men were found to already be HIV+, although none of them had clinical signs of AIDS. The table below describes the 10 subjects who either tested HIV positive, or became lost to follow-up, or already were HIV positive at the start of the study. Legend: HIV+ = found to be HIV+ at the very beginning of the interval observation period ? = lost to follow-up ************* Follow-up ********************** Subject # Initial Screening Jan. 2001 June 2001 Jan. 2002 June 2002 Jan. 2003 June 2003 1 --------- --------- --------- --------- --------- HIV+-- 2 --------- --------- --------- --------- --------- ? 3 --------- --------- HIV+-- --------- --------- --------- 4 --------- ? 5 --------- --------- --------- ? 6 --------- --------- --------- --------- --------- ? 7 --------- HIV+-- --------- --------- --------- --------- 8 --------- --------- --------- --------- --------- ? 9 HIV+ --------- --------- --------- --------- --------- --------- 10 HIV+ --------- --------- --------- --------- --------- ? What were the total person-months at risk during the study? a. 162 b. 1,242 c. 1,080 d. It is not possible to calculate total person-months with the information given.
Explanation / Answer
Out of the 40 patients, 30 were healthy and only 10 were found to be at risk
A subject is eligible to contribute person-time to the study only so long as that person does not yet have the health outcome under study and, therefore, is still at risk of developing the health outcome of interest
Given that,during the initial screening, 2 of the men were found to already be HIV+
So total number of subjects to be considered=10-2=8
The study was repeated every 6 months.
Subject 1: Contributed to 5 studies before being confirmed as HIV+.
Person-months=(5*6)=30 months
Subject 2:Contributed to 5 studies before being lost to follow up
Person-months=(5*6)=30 months
Subject 3: Contributed to 2 studies before being confirmed as HIV+.
Person-months=(2*6)=12 months
Subject 4: Contributed to 1 study before being lost to follow up
Person-months=(1*6)=6 months
Subject 5: Contributed to 3 studies before being lost to follow up
Person-months:(3*6)=18 months
Subject 6: Contributed to 5 studies before being lost to follow up
Person-months:(5*6)=30 months
Subject 7: Contributed to 1 study before being confirmed as HIV+
Person-months:(1*6)=6 months
Subject 8: Contributed to 5 studies before being lost to follow up
Person-months:(5*6)=30 months
Subject 9 and Subject 10 were not considered because they were confirmed as HIV+ at the initial screening
Total person-months at risk during the study=30+30+12+6+18+30+6+30=162
Option (a) is the answer
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