Detection of specific antigens present in the sample (blood, urine, CSF, or stool) using immunologic methods such as precipitin tests, particle agglutination tests, immunofluorescence, immunochromatographic or enzyme immunoassays are the mainstay for the rapid diagnosis of various infections. These test methods use polyclonal and monoclonal antibodies to detect specific antigens of the pathogen of interest.
Antigens are foreign substances, usually high molecular weight proteins or glycoproteins, that elicit the production of antibodies. One organism may contain different antigens that the host will recognize as foreign. Find more about antigens here
Numerous immunologic methods are used to detect bacteria, fungi, parasites, and viruses rapidly. For example,
- Ouchterlony double immunodiffusion method is used to detect exoantigens produced by systemic fungi to confirm their presence in culture.
- Latex agglutination tests are used to detect antigens to Cryptococcus neoformans in CSF or serum and to confirm the presence of beta-hemolytic Streptococcus from culture plates. Latex tests are also available to detect Streptococcus agalactiae, Clostridium difficile toxins A and B, and rotavirus.
- Coagglutination test is used for the identification of streptococci, including Lancefield groups A, B, C, D, F, G, and N; Streptococcus pneumoniae, Neisseria meningitidis; N.gonorrhoeae; and Haemophilus influenzae types A and F grown in culture.
- Immunofluorescent assays are popular tests for detecting antigens in clinical laboratories. Fluorescent antibody tests are commonly used to detect Bordetella pertussis, Legionella pneumophila, Giardia, Cryptosporidium, Pneumocystis, Trichomonas, adenovirus, cytomegalovirus, herpes simplex virus, influenza virus, parainfluenza virus, and varicella-zoster virus.
- Enzyme immunoassays (e.g. ELISA) are commonly used to detect Hepatitis B surface antigens (HBsAg), Hepatitis e antigen (HBeAg), HIV P24 antigen, and antigens of various infectious agents.
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Stool Antigen Test
Stool antigen tests (SATs) are noninvasive diagnostic modules for Helicobacter pylori infection. This test was introduced after urea breath test (UBT). The currently used stool antigen test for diagnosing H. pylori infection is based on enzyme immunoassay (EIA) or immunochromatography (ICA).
Urine Antigen Test
Urinary antigen testing has grown in popularity for several significant respiratory infections, particularly Legionella pneumophila (legionellosis), Streptococcus pneumoniae (pneumococcal pneumonia), and Histoplasma capsulatum (histoplasmosis). Though these organisms are not present in urine for culture, antigens shed by them get concentrated in the kidney and excreted in the urine. Soluble antigens from Streptococcus agalactiae and Haemophilus influenzae may concentrate in urine.
Urine antigens are then detected via an immunoassay such as an enzyme-linked immunosorbent assay (ELISA) or an immunochromatographic or lateral flow assay (LFA).
Antigens present in Cerebrospinal fluids (CSF)
Soluble capsular polysaccharide antigen produced by the most common etiologic agent of meningitis including the group B streptococcal polysaccharide are detected from CSF using latex agglutination, coagglutination, or commercial agglutination systems.
Polysaccharide capsular antigen of Cryptococcus neoformans can also be detected from CSF samples using commercial reagents.
Antigen testing in other samples:
- Detection of group A beta-hemolytic antigen from throat swabs
- Detection of group B streptococcal antigen in vaginal secretions
About the antigens:
Some commonly tested antigens during disease diagnosis are:
| Name of the Antigen | Disease |
| P24 | HIV Infection |
| Histidine rich protein 2 (HRP2) | Falciparum malaria |
| NS1 antigen | Dengue |
| HBsAg and HBeAg | Hepatitis B Infection |
| Parasite lactate dehydrogenase (pLDH). | Malaria |
| Aldolase | Malaria |
P24 Antigen
p24 is a capsid structural protein that makes up a protein ‘shell’ on the surface of the HIV virus. p24 test is generally only positive from about two to three weeks after infection with HIV.
Histidine rich protein 2 (HRP2)
HRP-II is an abundant protein expressed only by Plasmodium falciparum and is the target for the most commonly used RDTs.
Parasite lactate dehydrogenase (pLDH)
It is produced by the asexual and sexual stages (gametocytes) of malaria parasites. Malarial test kits that are currently available detect pLDH from all four species of Plasmodium. They can distinguish P. falciparum from the non-falciparum species, but cannot distinguish between P. malariae, P. ovale, and P. vivax.
HBeAg
HBeAg is a hepatitis B viral protein. It is an indicator of active viral replication; this means the person infected with Hepatitis B can likely transmit the virus to another person (i.e. the person is infectious).
HBsAg (Australia antigen)
HBsAg is the surface antigen of the hepatitis B virus (HBV). It indicates the current hepatitis B infection.
NS1 Antigen
NS1 tests detect the non-structural protein NS1 of dengue virus in the serum using synthetically labeled antibodies. NS1 is detectable during the acute phase of dengue virus infections.
NS1 tests can be as sensitive as molecular tests during the first 0-7 days of symptoms. After day 7, NS1 tests are not recommended. A positive NS1 test result is indicative of a dengue infection but does not provide serotype information.
References
- Urine Antigen Tests for the Diagnosis of Respiratory Infections. Clin Lab Med 34 (2014) 219–236 http://dx.doi.org/10.1016/j.cll.2014.02.002
- Bailey & Scott’s Diagnostic Microbiology, Forbes, 11th edition