IM6.9-12 | HIV Diagnostic Testing — Summary & Reflection

KEY TAKEAWAYS

HIV diagnostic testing follows a hierarchy: fourth-generation Ag/Ab combination assay (detects p24 antigen + antibodies, window period 18–45 days) → confirmatory HIV-1/HIV-2 differentiation assay. For acute infection or infant diagnosis: HIV RNA PCR (detectable day 10–12) or HIV DNA PCR (infants <18 months). CD4 count (flow cytometry) stages disease and guides prophylaxis thresholds; viral load monitors ART efficacy (target: undetectable by 6 months).

OI laboratory tests by site: Respiratory — CBC (anaemia in MAC), sputum CBNAAT/Xpert MTB/RIF (TB), serum LDH + (1→3)-β-D-glucan (PCP), BAL silver stain (PCP gold standard). Neurological — CSF India ink stain (Cryptococcus, ~70% sensitive), CSF/serum CrAg (>95% sensitive), CSF Toxoplasma PCR (low sensitivity, high specificity), CSF EBV PCR (PCNSL). GI — stool modified acid-fast stain (Cryptosporidium, Isospora), blood lysis-centrifugation culture (MAC). Disseminated — CMV DNA PCR, alkaline phosphatase (MAC).

Imaging: CXR — bilateral ground-glass (PCP), miliary (TB), nodular+effusion (KS), lobar consolidation (bacterial). CT brain — multiple ring-enhancing basal ganglia (toxoplasmosis), single large ring-enhancing (PCNSL), normal + raised ICP (cryptococcus), white matter hypodensity (PML). MRI brain — superior for small/posterior fossa lesions; target sign (toxoplasmosis); FLAIR white matter changes (PML, HIV encephalopathy).

ABG severity threshold for PCP corticosteroids: PaO₂ <70 mmHg OR A-a gradient >35 mmHg on room air → adjunctive prednisolone mandatory (reduces mortality ~50% in moderate-to-severe PCP).

REFLECT

Consider Patient A from the opening hook — SpO₂ 84%, bilateral hazy CXR, LDH 890 IU/L, HIV test pending. You now have the diagnostic framework to act. Without waiting for the HIV test result (which may take hours), the clinical picture already satisfies the threshold for empirical PCP treatment — bilateral ground-glass pattern, profound hypoxia, markedly elevated LDH in a patient at likely HIV risk. The ABG will determine the corticosteroid decision. Reflect on this question: in a district hospital with limited laboratory resources, if you had to choose only two investigations for this patient — one to guide acute treatment and one to guide long-term management — what would they be, and why? This single decision-making exercise encodes the clinical prioritisation that separates a skilled diagnostician from one who orders a panel of tests and waits.