How HIV is tested on Step 1
Each step of the life cycle is a drug target.
Diagnosis and disease course
Which test works depends on timing and age.
- Acute HIV: 2–4 weeks after exposure; fever, sore throat, rash, lymphadenopathy (mononucleosis-like); very high viral load. Antibody tests may still be negative.
- Clinical latency: years with few symptoms while CD4 cells steadily fall and the virus replicates in lymph nodes.
- AIDS: CD4 < 200 cells/µL or an AIDS-defining condition.
- Infants of mothers with HIV: use nucleic acid (PCR) testing, because maternal IgG crosses the placenta.
- Monitoring: viral load shows treatment response; CD4 count guides prophylaxis and staging.
Opportunistic infections and cancers
The CD4 count predicts which infections are likely.
| CD4 count (cells/µL) | Infection or cancer | Clue |
|---|---|---|
| Any count | Tuberculosis, oral candidiasis (thrush), HSV, VZV | TB can occur early |
| < 200 | Pneumocystis jirovecii pneumonia | Bilateral ground-glass opacities, high LDH; TMP-SMX prophylaxis |
| < 200 | Kaposi sarcoma (HHV-8) | Purple skin and mucosal lesions |
| < 150 | Histoplasmosis | Fever, weight loss, pancytopenia (Ohio and Mississippi River valleys) |
| < 100 | Toxoplasma gondii encephalitis | Multiple ring-enhancing lesions |
| < 100 | Cryptococcal meningitis | India ink, cryptococcal antigen |
| < 100 | Primary CNS lymphoma (EBV) | Single ring-enhancing lesion |
| < 50 | Mycobacterium avium complex | Fever, diarrhea, weight loss |
| < 50 | CMV retinitis, colitis, esophagitis | Cotton-wool retinal lesions; ganciclovir |
Progressive multifocal leukoencephalopathy (JC virus) causes white matter lesions without enhancement.
Antiretroviral drugs
Know each class and its signature toxicity.
Worked example
Use the CD4 count and imaging.
functions Worked example — headache and ring-enhancing lesions
A patient with untreated HIV and a CD4 count of 60 cells/µL has headache, fever, and confusion. MRI shows multiple ring-enhancing lesions in the basal ganglia. Toxoplasma IgG is positive. What is the diagnosis and treatment?
- At a CD4 count below 100, toxoplasmosis and primary CNS lymphoma are the main causes of ring-enhancing lesions.
- Multiple lesions and positive Toxoplasma serology favor toxoplasmosis; lymphoma is usually a single lesion.
- Treatment is pyrimethamine plus sulfadiazine, with leucovorin to protect the marrow.
- Lack of improvement in about 2 weeks would prompt biopsy for lymphoma.
Answer. Toxoplasma encephalitis; pyrimethamine, sulfadiazine, and leucovorin. TMP-SMX prophylaxis would have prevented it.