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PATIENT 
Mr. A asks his new primary care clinician whether he should get an HIV test. He states that he has “absolutely no risk factors for HIV,” and is a very healthy 21-year-old black man in a monogamous relationship with his girlfriend for 2 years. The girlfriend was tested for HIV 6 months ago when a Board of Health nurse notified her that she might have been exposed to HIV, and retested 3 months ago. Both HIV tests were negative, effectively ruling out HIV infection.
Mr. A first became sexually active at age 15. Over the last 6 years, he has had 4 female partners, with a bit of overlap between the relationships (he had 2 simultaneous partners for about 1 year). He has never had sex with a male. He uses condoms “pretty consistently.” He had Chlamydia trachomatis urethritis 3 years ago, but no other sexually transmitted infections. He has never used injecting drugs, although he smokes marijuana once or twice a week. He stopped drinking excessively when he met his current girlfriend. He does not recall an episode of mononucleosis-like illness with fever and lymph node enlargement. His past medical history, review of systems, and physical exam are otherwise unremarkable.
Is the clinical information sufficient to make a diagnosis? If not, what other information do you need?
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As noted above, 3 factors determine the positive predictive value of the test: the pretest probability, the sensitivity, and the specificity. Mr. A is asymptomatic and denies high-risk behaviors. However, he may not truly be at a low risk of HIV, since he had a prior sexually transmitted infection and did not always use a condom with his 4 sex partners. His history of alcohol binges may also point to forgotten prior high-risk behaviors. The history of simultaneous sexual partners is also a risk factor. His pretest probability of HIV infection is therefore significantly higher than he believes.
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A fourth-generation HIV-1/2 Ag/Ab combination immunoassay is positive. The HIV-1/HIV-2 Ab differentiation immunoassay confirmatory test detects HIV-1 Ab, confirming HIV-1 infection. Because of the excellent specificity of the combined tests, his posttest probability of HIV infection is > 99%. The CD4TL count is 150 cells/mcL (immunologic AIDS) and the ...