The term “serology” describes the use of antigen-antibody responses in the lab for diagnosis. Serum, the liquid portion of blood where antibodies are discovered, is employed in testing, hence the name. In the clinical laboratory, serologic testing may be utilized in two different ways: to discover unidentified antigens (such bacteria) and to find antibodies being produced against a certain antigen in the patient’s serum. Serologic testing comes in two different forms: direct and indirect.
Direct Serologic Testing
A preparation of known antibodies known as antiserum is used in direct serologic testing to identify an unknown antigen, such as a bacterium.
Indirect Serologic Testing
A known antigen is used in indirect serologic testing to identify antibodies in a person’s serum that were produced by that person against an antigen linked to a certain disease.
Differences Between Direct and Indirect Serologic Tests
The basis for direct serologic testing is the highly specific nature of antigen-antibody responses. Similar to an enzyme-substrate reaction, antibodies often only react with the antigen that initially stimulated their synthesis. As a result, one can employ known antiserum (made via the monoclonal antibody approach or animal inoculation, as explained above) to detect unknown antigens, such as bacteria.
Through a process called indirect serologic testing, antibodies generated by a person against an antigen linked to a specific disease are found in their serum and identified.
Based on the notion that antibodies can only be created in response to a particular antigen, this kind of testing. In other words, unless a disease antigen is present in the body and promoting antibody synthesis, a person won’t be making antibodies against it. In this kind of evaluation, The known antigen for that suspected disease is combined with a sample of the patient’s serum, which is the liquid fraction of the blood after it has clots and contains antibodies against the disease antigen if the person has the disease or has had it. After that, one looks for an antigen-antibody reaction.
Examples of serological tests to diagnose disease by the detection of antibodies in the patient’s serum include the various tests for syphilis or STS, tests for infectious mononucleosis, tests for the human immunodeficiency virus (HIV), tests for systemic lupus erythematosus, and tests for a variety of other viral infections.
Detailed Comparison Between Direct and Indirect Serologic Tests
|S.No.||Aspect||Direct Serologic Tests (DST)||Indirect Serologic Tests (IST)|
|1||Principle||Detects the presence of pathogen or its components||Detects antibodies produced in response to infection|
|2||Target||Pathogen-specific antigens or components||Antibodies produced against the pathogen|
|3||Test Format||Uses labeled antigens or antibodies||Uses labeled antibodies|
|4||Detection Method||Direct visualization of antigen or pathogen||Indirect detection of antibodies|
|5||Purpose||Detection of antigens or pathogen in samples||Detection and quantification of antibodies|
|6||Sensitivity||Often high sensitivity||Variable sensitivity|
|7||Specificity||Often high specificity||Variable specificity|
|8||False Positives||Can be prone to cross-reactivity||Less prone to cross-reactivity|
|9||False Negatives||Dependent on the target and method||May have false negatives due to low antibody levels|
|10||Antigen Source||Pathogen or antigen source is required||Antigen source is not required|
|11||Detection Time||Rapid results due to direct visualization||May require more time due to indirect detection|
|12||Diagnostic Range||Generally detects active infection||Often detects exposure history|
|13||Viral Load Measurement||Often not suitable for viral load measurement||Not suitable for viral load measurement|
|14||Presence of Antibodies||Not relevant to the test||Central to the test|
|15||Antibody Titer||Not applicable||Can be measured as antibody titer|
|16||Diagnostic Use||Often used for rapid diagnosis||Used for diagnostics, screening, and monitoring|
|17||Antigen-Antibody Complex||Directly visualized in positive cases||Not relevant to the test|
|18||Amplification Process||Generally does not involve amplification||Not applicable for direct tests|
|19||Maternal-Fetal Transfer||Can’t indicate maternal-fetal transfer||Can indicate maternal-fetal transfer|
|20||Tissue Involvement||Not usually involved in tissue diagnosis||Not relevant to the test|
|21||Point-of-Care Use||Commonly used for point-of-care testing||Less commonly used at the point of care|
|22||Diagnostic Window||Often detects infections during acute phase||May detect infections during acute or convalescent phase|
|23||Maturation Time||Rapid, as it involves direct detection||Slower, as it depends on antibody development|
|24||Multiplexing||Limited to detecting a specific target||Can be used for multiplexing|
|25||Quantitative Analysis||Not applicable for direct tests||Can provide quantitative data|
|26||Virus Mutation Detection||Limited ability to detect viral mutations||Not applicable for direct tests|
|27||Chronic Infections||Often detects acute infections||Can detect both acute and chronic infections|
|28||Sample Type||Requires samples with antigens or pathogens||Requires samples with antibodies|
|29||Abbreviation||DST (Direct Serologic Test)||IST (Indirect Serologic Test)|
1) Which serology tests are available, and how do they operate?
Qualitative serology tests provide a simple “yes” or “no” answer as to whether a subject has ever been exposed to SARS-CoV-2. The most popular quick serology test, lateral flow assays (LFAs), provides a qualitative readout of positive or negative results.
An individual’s “serostatus,” a phrase frequently used in public health to denote whether a person is positive or negative for the antibodies of interest, can be determined from this kind of test.
Chemiluminescent immunoassays (ChLIAs) and enzyme-linked immunosorbent assays (ELISAs), two quantitative serology procedures, provide more accurate results, such as the quantity of antibodies in a patient sample.
2) What varieties of serology tests are available?
Enzyme-linked immunosorbent assay (ELISA)
Rapid serology test (RST)
3) What is revealed by the neutralization assay?
It provides information on the existence of active antibodies in patient serum that can stop viral growth ex vivo in a cell culture system.
4) What are the fundamental issues with serological laboratory diagnosis?
Infectious disease serological laboratory diagnosis is plagued by a variety of fundamental issues. The twofold indirect nature of the serological detection of viral illnesses presents one challenge: Finding the microbiological agent that caused the illness is the primary indirect goal of diagnosing an infectious disease. By assessing the patient’s immune response to the probable agent, the second indirect goal is to identify this infectious agent. As a result, the serological test measures the patient’s immune system rather than the disease itself or its etiology.
5) What serologic test is required to confirm a syphilis diagnosis?
Nontreponemal tests and treponemal tests must be used in conjunction to diagnose and establish the stage of syphilis.