Infectious Diseases and Clinical Microbiology

Infectious Diseases: Diagnosis, Treatment & Travel Medicine

Infectious diseases specialists diagnose and treat infections from bacteria, viruses, fungi, and parasites, focusing on fevers, antibiotics, and vaccines.

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Methodologies and Clinical Assessment

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Diagnosing HIV has become much more advanced, moving from basic antibody tests to detailed molecular tests that can find the virus early. At Liv Hospital, the goal is to shorten the window period (the time between infection and when a test can detect HIV) and to give a full evaluation that helps plan long-term care. This process checks not just for the virus, but also looks at the patient’s immune system and the type of virus present.

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The Diagnostic Window and Testing Generations

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Diagnostic accuracy relies on understanding the biology of early infection. After HIV enters the body, distinct viral markers appear in the blood at different times. First, the viral RNA becomes detectable, followed by the p24 antigen (a viral protein), and finally, the host’s antibody response (IgM and IgG antibodies). Modern testing algorithms utilize “fourth-generation” assays to capture these markers as early as possible.

  • Nucleic Acid Tests (NATs): These tests look for the actual virus (RNA) in the blood. They are the most sensitive and can detect HIV 10 to 33 days after exposure. Due to their complexity, they are typically used to confirm acute infection or screen donated blood rather than for routine screening.
  • Antigen/Antibody Combination Tests: These are the standard of care for screening. They detect both HIV antibodies produced by the immune system and the p24 antigen, a viral protein. The p24 antigen appears before antibodies develop, allowing these tests to detect infection 18 to 45 days after exposure.
  • Antibody Differentiation Immunoassays: Once a screening test is positive, a confirmatory test is performed to differentiate between HIV-1 and HIV-2. This is crucial because the two viral types have different progressions and respond differently to certain medications.
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The Diagnostic Algorithm

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The process for diagnosing HIV uses a strict step-by-step approach to make sure results are accurate. It starts with a sensitive screening test, usually the antigen/antibody combination. If the test is negative, the person is considered uninfected. If it is positive, another test is done to tell the type of HIV. If the results do not match (for example, the first test is positive but the second is negative), a Nucleic Acid Test is used to check for early infection before antibodies appear. This careful process helps avoid false positives and ensures the diagnosis is correct.

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Baseline Evaluation and Staging

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Upon confirmation of HIV infection, a comprehensive medical evaluation is initiated to stage the disease and check for comorbidities. This sets the baseline against which the success of future treatment will be measured.

  • CD4 T-Lymphocyte Count: This measures the health of the immune system. A normal count ranges from 500 to 1,600 cells/mm³. This metric is used to determine the urgency of starting prophylactic medications against opportunistic infections and to stage the disease.
  • Quantitative Viral Load (HIV RNA): This measures the amount of virus in the blood. It provides a baseline to judge the effectiveness of antiretroviral therapy. A high baseline viral load may influence the choice of initial medication.
  • Genotypic Resistance Testing: Before starting treatment, the virus is analyzed for genetic mutations that might confer resistance to specific antiviral drugs. This ensures that the prescribed regimen will be fully effective against the particular strain of the virus the patient carries.

Pharmacogenomic Screening

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Modern HIV care involves personalized medicine. Specifically, screening for the HLA-B*5701 allele is mandatory before prescribing abacavir, a common antiretroviral drug. Patients who carry this genetic marker are at high risk for a severe, potentially life-threatening hypersensitivity reaction to the medication. This genetic screening prevents adverse events and tailors the drug regimen to the patient’s genetic profile.

Co-infection and Comorbidity Screening

Because HIV shares transmission routes with other pathogens and affects systemic health, the evaluation includes a broad screen for co-infections and organ health.

  • Viral Hepatitis: Screening for Hepatitis B and C is essential, as co-infection alters the progression of liver disease and influences drug selection (some HIV drugs also treat Hepatitis B).
  • Tuberculosis (TB): Latent TB screening is critical, as HIV increases the risk of latent TB becoming active.
  • Metabolic Profile: Kidney and liver function tests, lipid profiles, and fasting glucose are assessed to monitor for metabolic issues that the virus or its treatment can exacerbate.
  • Cardiovascular Risk Assessment: Given the increased inflammatory state associated with HIV, a thorough cardiovascular risk evaluation is conducted.

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FREQUENTLY ASKED QUESTIONS

What is the "window period" in HIV testing

The window period is the time between when a person is exposed to HIV and when a test can accurately detect the infection. During this time, a person may be infected and infectious but still test negative. The length of the window period depends on the type of test used; nucleic acid tests have the shortest window (10-33 days), while antibody tests have a longer window (23-90 days).

Multiple tests are used to ensure absolute accuracy. The initial screening test is designed to be extremely sensitive so it doesn’t miss any infections, but this can rarely lead to a false positive. A second, different type of test (a confirmatory test) is specific and is used to verify the first result. This two-step algorithm ensures that a diagnosis is correct before treatment begins.

Drug resistance testing analyzes the HIV genes found in a patient’s blood. It looks for specific mutations that would allow the virus to survive in the presence of certain antiviral medications. This information helps the doctor choose a medication combination that the virus is not resistant to, ensuring the treatment will successfully suppress the viral load.

HIV, Hepatitis B, and Hepatitis C share similar routes of transmission, such as sexual contact or blood exposure, so co-infection is common. Furthermore, HIV can accelerate the liver damage caused by hepatitis. Knowing the hepatitis status is vital because it changes the choice of HIV medications, as some drugs are active against both HIV and Hepatitis B.

The HLA-B5701 test is a genetic blood test performed before starting a patient on a specific HIV medication called abacavir. Some people have a genetic variation (the HLA-B5701 allele) that makes them allergic to this drug. If they take it, they can have a severe, dangerous reaction. Testing beforehand prevents this reaction by identifying who should not use abacavir.

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