The detection of toxin production by Corynebacterium diphtheriae in diagnostic laboratories

In vitro tests for assessing the toxigenicity of strains of C. diphtheriae using the original agar plate precipitin method of Elek are unreliable. At high concentrations of antitoxin (500-1000 U/ml) multiple non-specific precipitin lines occur making interpretation difficult. Increased specificity was obtained by diluting the antitoxin but this in turn caused a delay in the appearance of toxin-antitoxin lines. Also, the failure of lines to appear on media enriched with unsuitable serum may lead to false-negative results. Final assessment of the toxigenicity of strains of C. diphtheriae can probably only be made reliably by guinea-pig tests.