Antigen Detection Tests for Disease Diagnosis: Methods, Examples, and Clinical Uses

A 32-year-old man presents on day 2 of fever, severe headache, and neck stiffness. CSF is collected by lumbar puncture. A Gram stain shows no organisms. Culture will take 48–72 hours. The patient is critically ill and cannot wait.

In the laboratory, a latex agglutination test for capsular polysaccharide antigens is performed on the CSF in 15 minutes and returns positive for Cryptococcus neoformans. Antifungal treatment begins immediately. The culture confirms Cryptococcus three days later.

This case captures the defining advantage of antigen detection: it identifies the pathogen directly, without waiting for the immune system to respond. In early infection, in immunocompromised patients who cannot mount adequate antibody responses, and in any situation requiring a same-visit result, antigen detection frequently provides diagnostic answers that antibody-based serology cannot.

This article and its companion: This article covers antigen detection — identifying pathogen components directly in clinical specimens. For antibody-based serological tests (ELISA, RPR, TPHA, Widal, ASO, Monospot, and more), see the companion article: Serological Tests for Disease Diagnosis.

Together these two articles cover the full spectrum of immunodiagnostic testing. The fundamental difference: antigen tests ask "Is the pathogen here right now?"; antibody tests ask "Has the body responded to this pathogen?"

Detection of specific antigens in a sample (blood, urine, CSF, or stool) using immunologic methods; precipitin tests, particle agglutination, immunofluorescence, immunochromatography, or enzyme immunoassay is the mainstay of rapid infection diagnosis. 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.

Why Antigen Detection? The Clinical Case

In any infection, antigen precedes antibody. The pathogen (or its proteins) enters the host before the immune system has time to generate a detectable antibody response. This creates a window period during which antibody tests are negative even though infection is present.

Antigen detection closes this window. It also solves three other diagnostic challenges that antibody testing cannot:

The practical rule: Use antigen detection first when the patient is acutely ill and early in the illness. Switch to or add antibody detection when the clinical picture is subacute, retrospective, or when immunity status is the question (post-vaccination, epidemiology).

Methods Used in Antigen Detection

The following methods are used to detect pathogen antigens in clinical specimens. Each has different sensitivity, specificity, speed, and infrastructure requirements:

Figure: Blood sample and lab request for anti-HIV testing

Stool Antigen Test

Stool antigen tests (SATs) are noninvasive diagnostic tools for Helicobacter pylori infection, introduced after the urea breath test (UBT). 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).

Stool antigen test (SAT) is recommended by WHO for both initial diagnosis and test-of-cure after H. pylori eradication therapy. For test-of-cure, SAT should be performed ≥4 weeks after completing antibiotic treatment. Serology (anti-H. pylori IgG) is not suitable for test-of-cure because IgG remains elevated for months to years after successful eradication.

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 generally cannot be cultured from urine, antigens shed by them are concentrated in the kidney and excreted in 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).

When urine antigen testing changes clinical management: The Legionella urinary antigen test (UAT) detects Legionella serogroup 1 responsible for ~80% of Legionnaires' disease within 3 hours. It allows targeted antibiotic therapy (levofloxacin or azithromycin instead of broad-spectrum) at a stage when culture results are not yet available. Similarly, the pneumococcal UAT can guide de-escalation from broad-spectrum coverage in severe community-acquired pneumonia within the first 24 hours.

Antigens present in Cerebrospinal fluid (CSF)

Soluble capsular polysaccharide antigens produced by the most common bacterial agents of meningitis including Streptococcus pneumoniae, Neisseria meningitidis, Haemophilus influenzae type b, and group B Streptococcus can be detected in CSF using latex agglutination, coagglutination, or commercial agglutination systems.

Cryptococcal antigen- the clinical priority: Cryptococcal meningitis is the most common cause of meningitis in HIV-positive adults in sub-Saharan Africa, responsible for 15–20% of AIDS-related deaths. WHO recommends Cryptococcal antigen (CrAg) lateral flow assay screening in all HIV-positive patients with CD4 <100 cells/µL before they develop symptoms. The CrAg LFA is >99% sensitive and specific, costs less than $5, and requires no laboratory equipment. Early detection at the asymptomatic stage dramatically reduces mortality.

Antigen testing in other samples

• Detection of group A beta-hemolytic antigen from throat swabs
• Detection of group B streptococcal antigen in vaginal secretions

Commonly Tested Antigens in Disease Diagnosis

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.

Figure: Microbiologist standing at desk and holding test tube rack while preparing blood samples for centrifuge

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.

How to Remember

Antigen = the pathogen's ID card; antibody = the body's response to seeing the ID card. Antigen tests look for the ID card directly. Antibody tests look for proof the body has seen it. The ID card appears first — which is why antigen tests work earlier in infection.

The three specimen types and what they tell you:

The rapid test revolution in LMIC settings: Malaria RDTs, dengue NS1 RDTs, HIV combo tests, Cryptococcal LFAs, and COVID-19 antigen tests all share one characteristic — they are lateral flow immunochromatographic tests that any trained health worker can perform in 15 minutes without laboratory equipment. These RDTs have transformed diagnostics in district hospitals and health posts where culture and ELISA are not available. Understanding the antigen each detects and its clinical window is the core knowledge for any healthcare worker in tropical settings.

The window period visual:

DAY 0         DAY 3–5          DAY 7–14         WEEKS–MONTHS
  |               |                |                  |
Infection    ANTIGEN peaks    IgM appears        IgG persists
              (detectable)    (seroconversion)   (for life)

Use: ← Antigen tests here → ← IgM tests here → ← IgG tests here →

The earlier the presentation, the more you need antigen detection. The later or more retrospective the diagnosis, the more antibody tests help.

Key exam facts in one table

References

1. Tille, P. M. (2017). Bailey and Scott's Diagnostic Microbiology (14th ed.). Elsevier.
2. World Health Organization. (2022). Guidelines for the Diagnosis, Prevention and Management of Cryptococcal Disease in HIV-infected Adults, Adolescents and Children. Geneva: WHO.
3. World Health Organization. (2015). Guidelines for the Treatment of Malaria (3rd ed.). Geneva: WHO. [Malaria RDT section]
4. Huong, V. T. Q., et al. (2014). Urine antigen tests for the diagnosis of respiratory infections. Clinics in Laboratory Medicine, 34(2), 219–236.
5. Leber, A. L. (Ed.). (2016). Clinical Microbiology Procedures Handbook (4th ed.). ASM Press.