In immunofluorescence (IF) techniques, the direct versus indirect method differ in the number of antibody layers used. Which statement correctly distinguishes direct from indirect immunofluorescence in diagnostic microbiology?

• A Direct IF uses two antibody layers and is more sensitive than indirect IF
• B Indirect IF requires antigen immobilisation on a glass slide; direct IF is performed in solution
• C Direct IF uses FITC-labelled antigen to detect patient serum antibodies; indirect uses HRP-labelled antibody
• D Direct IF uses a single fluorochrome-labeled primary antibody against the antigen (e.g., DFA for C. trachomatis); indirect IF uses an unlabelled primary antibody followed by a fluorochrome-labeled anti-species secondary antibody — indirect is more sensitive due to signal amplification ✓

Explanation

Direct immunofluorescence (DFA): a single step using fluorochrome (FITC)-conjugated antibody directly against the microbial antigen — used for C. trachomatis DFA, RSV DFA, rabies diagnosis (DFIT on brain impression smear). Indirect immunofluorescence (IFA): two steps — unlabelled primary antibody binds antigen first, then fluorochrome-labelled anti-species secondary antibody binds the primary. IFA amplifies the signal (multiple secondary antibodies bind one primary), making it more sensitive. IFA is widely used in serology (e.g., ANA-IFA for autoimmune disease, Leishmania IF).

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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