29 Differences Between Direct and Indirect Serologic Tests

Introduction

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)

FAQ’S

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)

Neutralization assay

Chemiluminescent immunoassay

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.