Lab Tests Reference Library

HBsAg (Hepatitis B Surface Antigen) is the first serological marker to appear after Hepatitis B virus (HBV) infection, detectable 1–10 weeks after exposure — before symptoms or liver enzyme abnormalities appear. A positive HBsAg indicates that Hepatitis B virus is present and the person is infectious. It is the standard screening test for Hepatitis B infection worldwide.

Anti-HBs (Hepatitis B Surface Antibody) indicates immunity to Hepatitis B — either from successful vaccination or from natural recovery after infection. It is the only antibody produced after vaccination and is the marker used to assess vaccine-induced immunity. A level of ≥10 IU/L is considered protective immunity against Hepatitis B.

Anti-HCV (Hepatitis C Antibody) detects antibodies produced against Hepatitis C virus (HCV). It is the standard first-line screening test for Hepatitis C infection. A positive (reactive) result requires confirmation with HCV RNA PCR because Anti-HCV remains positive for life — even after the infection has been successfully treated and cleared.

Fourth-generation HIV combination test simultaneously detects HIV-1 and HIV-2 antibodies AND HIV-1 p24 antigen. Detection of both antigen and antibody dramatically shortens the diagnostic window — detecting HIV infection as early as 14–18 days post-exposure, weeks before antibody-only tests would be positive. This is the recommended standard for HIV screening.

CD4+ T cells (helper T cells) are the primary target of HIV. The CD4 count measures the number of CD4+ T lymphocytes per microliter of blood and reflects the current state of immune function in HIV-positive patients. It is the most important monitoring tool for HIV disease progression and is used to guide antiretroviral therapy initiation and opportunistic infection prophylaxis.

Dengue NS1 (Non-structural protein 1) antigen is a viral glycoprotein secreted at high concentrations into the bloodstream during early dengue virus infection. It is detectable from Day 1 to Day 5 of fever — before antibodies appear. NS1 testing is the most sensitive early diagnostic test for dengue fever during the acute febrile phase and helps distinguish dengue from other febrile illnesses.

Dengue IgM antibody appears 4–5 days after the onset of dengue fever and persists for 2–3 months. A positive IgM indicates recent dengue infection and is the test of choice after Day 5 of fever when NS1 antigen has already been cleared. Dengue IgM serology complements NS1 testing to ensure accurate diagnosis throughout the course of dengue illness.

The Widal test is an agglutination test that detects antibodies (IgM and IgG) against Salmonella typhi and Salmonella paratyphi antigens — specifically the O (somatic/cell wall) and H (flagellar) antigens. It has been used for over 130 years to diagnose typhoid fever in endemic areas where blood culture is unavailable. However, it has significant limitations in both sensitivity and specificity.

Malaria Rapid Diagnostic Test (RDT) detects Plasmodium antigens in a finger-prick blood sample within 15–20 minutes. Most malaria RDTs detect HRP-2 (histidine-rich protein 2) specific to Plasmodium falciparum — the most dangerous malaria species — and/or pLDH (parasite lactate dehydrogenase) which detects P. vivax, P. malariae, and P. ovale. RDTs have replaced microscopy in many endemic areas due to speed, ease of use, and not requiring laboratory infrastructure.