Elek’s test is an in-vitro gel (double) immunodiffusion test used to detect toxigenicity of Corynebacterium diphtheriae — i.e., whether an isolated strain actually produces diphtheria exotoxin, which is the true determinant of pathogenicity (non-toxigenic strains do not cause the classical disease).
Principle and procedure: A strip of filter paper soaked in diphtheria antitoxin is embedded in a Petri dish of serum agar. The test organism (and known toxigenic and non-toxigenic control strains) are streaked at right angles to the paper strip. The plate is incubated at 37°C for 24–48 hours. As the organism grows, secreted toxin diffuses through the agar and meets antitoxin diffusing from the strip; where toxin and antitoxin meet in optimal proportions, a visible line of precipitation forms at an angle to the streak.
Interpretation: Formation of a precipitin line (continuous with the line from the known toxigenic control) indicates the strain is a toxin producer; absence of a line indicates a non-toxigenic strain. This confirms toxigenicity independent of biotype (gravis/intermedius/mitis), which is determined separately by colony/biochemical characteristics.
