What is the recommended diagnostic algorithm for HIV infection in individuals of any age, including screening, confirmatory testing, and testing in infants under 18 months?

HIV Diagnostic Testing Algorithm

Initial Screening Test

Begin with a fourth-generation HIV antigen/antibody combination assay as the initial screening test for all patients. 1, 2, 3

• Fourth-generation assays detect both HIV antibodies (IgM and IgG) and p24 antigen simultaneously, allowing detection of acute infection approximately 2 weeks earlier than antibody-only tests 1, 2, 4
• If fourth-generation testing is unavailable, use a rapid HIV test or conventional ELISA, though these are less sensitive for acute infection 2
• Rapid tests can be performed on whole blood, plasma, serum, or oral fluid specimens, though oral fluid tests have higher false-positive rates 2

Confirmatory Testing Algorithm

All reactive screening tests must be confirmed before diagnosis—never diagnose HIV based on screening alone. 1, 2

Step 1: If Initial Screen is Reactive

• Perform HIV-1/HIV-2 antibody differentiation immunoassay to confirm infection and distinguish between HIV-1 and HIV-2 1, 2, 3

Step 2: Interpret Differentiation Assay Results

• If differentiation assay is positive: HIV infection is confirmed 1
• If differentiation assay is negative or indeterminate: Proceed immediately to HIV RNA nucleic acid amplification test (NAAT) to rule out acute HIV-1 infection 1, 2

Step 3: HIV RNA Testing

• A positive HIV RNA test confirms acute HIV infection even with negative antibody results 1, 2
• If HIV RNA is negative and differentiation assay was negative, the person is considered uninfected unless acute infection is still suspected 1

Traditional Algorithm (Still Valid Alternative)

• Initial screening with enzyme immunoassay (EIA/ELISA) for HIV-1/HIV-2 antibodies 1
• If repeatedly reactive, perform confirmatory Western blot 5, 1
• Positive Western blot confirms HIV infection (sensitivity and specificity ≥99%) 5
• Indeterminate Western blot requires follow-up testing at 4-6 weeks and consideration of HIV RNA testing 1

Special Population: Infants Under 18 Months

Standard antibody tests are unreliable in infants <15-18 months due to transplacental passage of maternal HIV antibodies. 5, 1, 2

• Definitive diagnosis requires two positive HIV RNA PCR or viral culture tests on separate specimens 1, 2
• Perform HIV RNA testing at ≤1 month of age and again at ≤4 months of age 5
• If any test is positive, repeat testing to confirm diagnosis 5
• Passively acquired maternal antibody falls to undetectable levels by 15 months of age in most uninfected infants 5

HIV-2 Testing Considerations

Test for HIV-2 in persons from endemic regions (West Africa, Angola, Mozambique, Portugal, France) or their sexual partners. 5, 1

• Also test for HIV-2 when there is clinical evidence of HIV disease but negative HIV-1 antibody tests 5, 1
• If HIV-2 EIA is repeatedly reactive with negative/indeterminate HIV-1 Western blot, send specimen to state public health laboratory for HIV-2 supplemental testing 1

Acute HIV Infection Protocol

If clinical suspicion exists for acute HIV infection (fever, lymphadenopathy, rash, recent high-risk exposure), perform HIV RNA testing immediately even if antibody tests are negative. 2

• Antibody tests cannot rule out infection that occurred less than 6 months before testing, as HIV antibody is detectable in ≥95% of patients within 6 months of infection 5, 1
• Fourth-generation assays can detect infection 11 days earlier than third-generation assays 6, 7

Critical Pitfalls and Caveats

The "Second Diagnostic Window"

• A rare phenomenon can occur where fourth-generation assays initially detect p24 antigen, then become negative as p24 antigen declines below detection limits before antibodies appear 6, 8
• During this second window, neither third nor fourth generation assays are reactive 6
• If acute infection is suspected but tests are negative, repeat testing in 2-4 weeks 6, 8

PrEP Users

• Pre-exposure prophylaxis (PrEP) use can attenuate or delay HIV seroconversion in diagnostic assays 1
• Reactive results in PrEP users may represent true infection or false positives 1
• Management depends on pre-test probability: stop PrEP for 3-4 weeks and retest, or intensify to full antiretroviral therapy if clinical suspicion is high 1

Consent Requirements

• Informed consent must be obtained before HIV testing (some states require written consent) 5, 1
• Opt-out screening is recommended where patients are informed testing will be performed unless they decline 5

Never Disclose Without Confirmation

• False positives can occur with screening tests—never disclose a positive HIV diagnosis without confirmatory testing 1
• The rate of unspecific reactivities is slightly higher in fourth-generation assays compared to conventional antibody assays (1.1% vs. 0.4%) 7

Post-Diagnosis Requirements

All persons with confirmed positive HIV tests must receive immediate medical and psychosocial evaluation or be referred for these services. 5, 1, 2

• Obtain baseline CD4 count, HIV RNA viral load, and genotype resistance testing before initiating therapy 2, 3
• Screen for sexually transmitted infections, hepatitis B and C, and tuberculosis 2, 3
• Provide behavioral and psychosocial services as an integral part of HIV care 5, 1, 2
• Initiate antiretroviral therapy immediately upon diagnosis, regardless of CD4 count or viral load 2, 3