Enzyme-Linked Immunosorbent Assay (ELISA) Test: Purpose and Interpretation in Diagnosing Infections
The Enzyme-Linked Immunosorbent Assay (ELISA) test is a highly sensitive and specific immunological assay used to detect antigens or antibodies in patient samples, serving as a valuable diagnostic tool for identifying active infections or previous exposure to pathogens, with different formats offering varying applications in clinical practice.
Basic Principles and Types of ELISA
ELISA is a rapid and sensitive method for detection and quantitation of an antigen using enzyme-labeled antibodies or for detecting antibodies against specific antigens 1.
Four basic ELISA formats exist, each with specific applications:
- Direct ELISA: Simplest format requiring an antigen and an enzyme-conjugated antibody specific to the antigen 1.
- Indirect ELISA: Detects antibodies in patient samples using antigen-coated plates and enzyme-linked secondary antibodies 2.
- Sandwich ELISA: Uses antibody-coated plates to capture antigens, followed by detection antibodies 3.
- Competitive ELISA: Involves competition between sample antigen and plate-coated antigen for antibody binding 3.
Clinical Applications in Infection Diagnosis
ELISA tests are widely used for detecting antibodies against infectious agents, with sensitivity and specificity generally exceeding 98% for well-validated tests 4.
For bacterial infections like Helicobacter pylori, serological ELISA tests detect specific anti-H. pylori IgG antibodies in serum but cannot distinguish between active infection and previous exposure 5.
In fungal infections, ELISA can detect specific antigens such as mannan (Mn) from Candida species or antibodies against these antigens (A-Mn), helping diagnose invasive candidiasis 5.
For viral infections, ELISA is commonly used as the initial screening test, with positive results typically confirmed by more specific tests like Western blot (as in HIV testing) 4.
Strengths and Limitations in Clinical Practice
Strengths:
High sensitivity and specificity when properly validated 4, 6.
Versatility in detecting either antigens (direct evidence of pathogen presence) or antibodies (indirect evidence of immune response) 5.
Relatively rapid results compared to culture-based methods 3.
Ability to process large numbers of samples, making it suitable for epidemiological studies 7.
Limitations:
Antibody detection cannot reliably distinguish between active infection and past exposure, as antibodies may persist in blood long after infection resolution 5, 7.
For H. pylori diagnosis, serological ELISA tests have limited accuracy (averaging only 78%) compared to other diagnostic methods like urea breath test or stool antigen test 5, 7.
False-positive and false-negative results can occur due to various factors including cross-reactivity with other pathogens 5.
Cannot be reliably used to confirm eradication of certain infections (like H. pylori) after treatment 7.
Interpretation Guidelines for Different Infections
Bacterial Infections (e.g., H. pylori):
Positive antibody ELISA indicates either current infection or previous exposure 5.
For H. pylori, stool antigen tests or urea breath tests are preferred over serology for diagnosing active infection and confirming eradication 7.
Serology may be useful in epidemiological surveys but has limited clinical utility due to inability to distinguish active from past infection 5, 7.
Fungal Infections:
For invasive candidiasis, combined detection of mannan antigen (Mn) and anti-mannan antibodies (A-Mn) by ELISA improves diagnostic accuracy 5.
In cryptococcal infections, antigen detection in blood and/or CSF by ELISA has high sensitivity (>95%) for diagnosing cryptococcal meningitis 5.
Viral Infections:
For HIV, ELISA is typically used as an initial screening test with positive results confirmed by Western blot 4.
Interpretation should consider the window period between infection and antibody development 4.
Special Considerations and Pitfalls
Recent antibiotic use or immunosuppression may affect antibody production and lead to false-negative results 5, 7.
Proton pump inhibitors should be discontinued before testing for H. pylori to avoid false-negative results 7.
Cross-reactive immune responses may affect specificity, particularly in closely related pathogens 5.
In transplant medicine, ELISA methods for detecting anti-HLA antibodies have been shown to be more predictive of post-transplant rejection than membrane-dependent assays 5.
Near-patient or rapid ELISA tests generally have lower sensitivity and specificity than laboratory-based tests 5, 7.
Recommended Testing Algorithms
For H. pylori Diagnosis:
- First choice: 13C-urea breath test or laboratory-based monoclonal stool antigen test 7.
- If patient has recently used antibiotics or PPIs: Consider validated IgG serology or wait 2 weeks after stopping medications 7.
- For patients requiring endoscopy: Perform rapid urease test, histology, or culture 7.
- For confirmation of eradication: Use UBT or stool antigen test at least 4 weeks after treatment completion; do not use serology 7.