The in vitro test that detects diphtheria toxin production by precipitating toxin and antitoxin in agar gels is known as the?

• A. D-test
• B. Elek test
• C. Nagler test
• D. Modified Hodge test

Answer: (B)

The main idea is detecting toxin production through immunodiffusion in a gel. In the Elek test, a toxin-producing strain of Corynebacterium diphtheriae releases diphtheria toxin into the surrounding agar that contains diphtheria antitoxin. Where the diffusing toxin meets its specific antitoxin in the gel, they form an insoluble complex that precipitates, producing a visible line. A positive result shows a line of precipitation at right angles to the bacterial streak, indicating the organism can produce toxin. If the organism does not produce toxin, no such line forms.

This test specifically targets toxigenicity, distinguishing toxigenic from non-toxigenic strains. Other tests mentioned assess different properties—antibiotic resistance in one case, or toxin-related activities of other organisms in different contexts—so they don’t identify diphtheria toxin production in this in vitro setup.