When someone receives a positive result on an initial HIV test, it sets off a series of critical steps in the medical microbiology lab to confirm the diagnosis and assess the state of the infection. These steps help guide treatment decisions, monitor disease progression, and protect public health. In this blog post, we’ll walk through the laboratory processes that follow a positive HIV screening result, including confirmatory testing, Western blot (historically), and viral load testing.
1. Initial HIV Screening: What Triggers the Next Steps
The journey typically begins with an HIV screening test, most often an enzyme-linked immunosorbent assay (ELISA) or fourth-generation HIV antigen/antibody test. These tests are highly sensitive and designed to pick up even early HIV infections by detecting:
- HIV antibodies (produced by the immune system)
- p24 antigen (a part of the virus itself)
A positive result here doesn’t confirm HIV on its own—it only indicates that further testing is needed.
2. Confirmatory Testing: Ensuring Accurate Diagnosis
To rule out false positives, labs perform confirmatory testing using more specific assays. Historically, the Western blot test was the gold standard, but it has largely been replaced in modern labs due to improvements in diagnostic technologies.
A. HIV-1/HIV-2 Differentiation Immunoassay (Recommended Method)
This is now the preferred confirmatory test in most settings. It determines:
- If the patient has HIV-1 or HIV-2
- Whether the antibodies detected in the screening test are specific to HIV
B. Western Blot Test (Historical but Still Used in Some Labs)
- Detects antibodies to individual HIV proteins.
- A positive result requires bands corresponding to multiple viral proteins.
- While once the standard, Western blot is slower, more expensive, and more prone to indeterminate results compared to newer methods.
If confirmatory testing is positive, the person is officially diagnosed with HIV. If inconclusive, further testing or repeat testing after a few weeks may be necessary.
3. HIV RNA (Viral Load) Testing: Measuring the Virus in the Body
Once HIV infection is confirmed, the lab performs an HIV viral load test, which:
- Measures the amount of HIV RNA (genetic material) in the blood
- Helps assess how actively the virus is replicating
- Provides a baseline for monitoring treatment
Viral load testing is essential for:
- Staging the infection
- Guiding treatment decisions (e.g., when to start antiretroviral therapy)
- Monitoring therapy effectiveness over time
A high viral load means active viral replication and possibly recent infection. A low or undetectable viral load (after treatment) means the virus is suppressed, though not cured.
4. CD4 Count Testing: Evaluating Immune System Health
Often performed alongside viral load testing, CD4 count measures the number of CD4+ T-cells, which are targeted by HIV. This tells clinicians:
- How advanced the infection is
- Whether the patient is at risk for opportunistic infections
- The need for prophylactic treatments
5. Drug Resistance Testing (Genotyping and Phenotyping)
Before or soon after treatment begins, labs may perform HIV resistance testing to detect mutations in the virus that may make it resistant to certain antiretroviral drugs. This helps in:
- Choosing the most effective drug regimen
- Avoiding treatment failure
6. Ongoing Monitoring: A Partnership Between Lab and Clinician
Medical microbiology labs play an ongoing role in patient management by:
- Repeating viral load and CD4 count testing every 3–6 months
- Alerting healthcare providers to any treatment failure
- Supporting public health surveillance
Conclusion
A positive HIV test is not the end—it’s the beginning of a structured, science-based process in the laboratory that confirms the infection, characterizes the virus, and supports life-saving treatment. Today’s medical microbiology labs use a combination of sophisticated immunoassays, molecular diagnostics, and monitoring tools to ensure every patient receives accurate results and timely care. By understanding what happens after a positive HIV test, patients and clinicians can work together to achieve the best possible outcomes.