In clinical microbiology and serology: The Fluorescent Treponemal Antibody (FTA-ABS) test is an example of which immunofluorescence technique?
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AIndirect immunofluorescence
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BDirect immunofluorescence
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CBoth direct and indirect immunofluorescence
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DNone of these
Answer
Correct Answer: Indirect immunofluorescence
Explanation
Introduction / Context:The Fluorescent Treponemal Antibody Absorption test (FTA-ABS) is a confirmatory serologic assay for syphilis caused by Treponema pallidum. Knowing whether it is a direct or indirect immunofluorescence method is essential for interpreting its workflow, sensitivity, and specificity in diagnostic microbiology.
Given Data / Assumptions:
- The assay detects anti–T. pallidum antibodies in patient serum.
- Fluorochrome-labeled anti-human immunoglobulin is available for visualization.
- Treponemal antigen is affixed to a slide.
Concept / Approach:
In direct immunofluorescence, a fluorochrome-labeled antibody binds directly to its antigen. In indirect immunofluorescence, an unlabeled primary antibody from the patient first binds to antigen; a labeled secondary anti-immunoglobulin then binds to that primary antibody to generate the fluorescent signal. The FTA-ABS uses the patient's serum antibodies as the primary (unlabeled) layer and a labeled anti-human Ig as the secondary layer, which is the defining feature of indirect immunofluorescence.
Step-by-Step Solution:
1) Immobilize treponemal antigen on a glass slide.2) Incubate patient's serum; if anti-treponemal antibodies are present, they bind the antigen.3) Wash to remove unbound serum components.4) Add fluorescein-labeled anti-human Ig (secondary antibody) that binds patient Ig attached to the antigen.5) Examine under a fluorescence microscope for apple-green fluorescence, indicating a positive reaction.Verification / Alternative check:
Indirect immunofluorescence generally provides signal amplification because multiple secondary antibodies can bind a single primary antibody, increasing sensitivity—consistent with the FTA-ABS test's high sensitivity in later stages of syphilis. Control slides with known positive and negative sera are run in parallel to verify performance.
Why Other Options Are Wrong:
- Direct immunofluorescence: would require a fluor-labeled antibody to T. pallidum applied directly to the antigen; FTA-ABS does not do this.
- Both direct and indirect: FTA-ABS is specifically indirect; it is not a hybrid protocol.
- None of these: incorrect because indirect is correct.
Common Pitfalls:
- Confusing screening (non-treponemal) tests with treponemal confirmatory tests.
- Assuming “fluorescent” always means direct labeling of the pathogen; in FTA-ABS the label is on the secondary anti-human Ig.
Final Answer:
Indirect immunofluorescence