HIV Testing: How to Calculate Infected Blood Risk, Video Explanation

Are you preparing for your Pathology or Internal Medicine Certifying Exam? If you are, you have likely run across some different practice questions regarding HIV Testing. One of our questions on this topic from our Pathology and Internal Medicine Board Review Question Banks is particularly tricky, so we’ve gotten our partner, MedCram to explain it in a recent video.

Check out the question:

In a move that “could increase the nation’s blood supply by at least 4% a year,” the FDA has recommended an end to a lifetime ban on blood donations for gay men. The FDA issued draft guidance in 2015 regarding the acceptance of men who have sex with men (MSM) to donate blood. The window for HIV-NAT (nucleic acid testing) is approximately 9 days, and the incidence of HIV in MSM is about 1% per year (i.e. 1 per 100 persons/year). Based on this information, what is the likelihood that an MSM individual would be within the window to transmit HIV via component?

A. 1/1,500,000
B. 1/4,055 (Correct)
C. 1/357,000
D. 1/100,000
E. 1/507

Correct answer: (B) 1/4,055.

Explanation: HIV incidence rates occur from 1% to as high as 8% in some high-risk populations such as urban MSM. If an MSM from a low-risk population with a 1% incidence (1/100 persons/year) of HIV donated blood, the likelihood that this individual would be in the window would be calculated as follows: Length of window period x Incidence of infection in the donor population = (9 days/365 days) x (1/100 persons/year) = 1/4,055. (A) 1/1,500,000 is the likelihood the general donor population would donate blood in the window and the product would transmit HIV. The risk in the MSM population is clearly much higher than in the general donor population. (C) 1/357,000 is the likelihood of transmission of Hepatitis B virus in the general donor population. (D) 1/1,100,000 is the likelihood of transmission of Hepatitis C virus in the general donor population. (E) 1/507 is the likelihood that an MSM individual from a high-risk urban area would be in the window to transmit HIV via component.

Reference: Roback JD, et al. AABB Technical Manual, 17th ed. Bethesda, MD: AABB. 2011. p. 254-255.
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