A 32-year-old man presents to the clinic with a 3-month history of fever, weight loss, and oral candidiasis. He is suspected to have HIV infection. Which investigation is most appropriate for initial screening and confirmation of HIV status?

Explanation

## Initial Screening for HIV Infection **Key Point:** The 4th generation antigen-antibody combination immunoassay (p24 antigen + anti-HIV-1/HIV-2 antibodies) is the recommended initial screening test for HIV in resource-limited and resource-rich settings. ### Why 4th Generation Assay? **High-Yield:** The 4th generation assay detects: - HIV-1 and HIV-2 antibodies - p24 antigen (viral capsid protein) - Reduces the window period to ~18 days post-infection - Sensitivity and specificity both >99.5% ### Testing Algorithm in India ```mermaid flowchart TD A[Suspected HIV]:::outcome --> B[4th Gen Ag-Ab Immunoassay]:::action B --> C{Result?}:::decision C -->|Positive| D[Confirmatory test<br/>HIV-1/HIV-2 differentiation<br/>or RNA PCR]:::action C -->|Negative| E[Window period?<br/>Repeat at 4 weeks]:::decision D --> F[Confirmed HIV+]:::outcome ``` ### Role of Other Tests | Test | Timing | Purpose | |------|--------|----------| | **4th Gen Ag-Ab** | Initial screening | First-line detection | | **HIV RNA PCR** | Post-positive screening | Confirmation, viral load quantification | | **CD4 count** | After confirmation | Staging, ART initiation decision | | **Western blot** | Rarely used now | Outdated; poor sensitivity in acute infection | **Clinical Pearl:** The window period (time between infection and test positivity) is shortest with 4th generation assays, making them superior to 3rd generation (antibody-only) tests. **Mnemonic: CARE** — **C**ombination (Ag + Ab), **A**ntigen (p24), **R**apid (shortest window), **E**arly detection (18 days). [cite:Harrison 21e Ch 197]