ROLE OF SODIUM-AZIDE IN REDUCING NONSPECIFIC COLOR DEVELOPMENT IN ENZYME IMMUNOASSAYS

Improved enzyme immunoassay (EIA) procedures achieved by incorporating sodium azide during predilution of serum samples in a solid-phase EIA for the detection of anti- Toxoplasma antibody in swine using a perox idase conjugate and in all washes of a bovine brucellosis rapid card t est EIA using alkaline phosphatase conjugate are reported. Without thi s modification, substantial background interference was encountered th at showed direct correlation with the degree of hemolysis of the serum samples. Anti-Toxoplasma gondii antibody-negative samples, separated by subjective groupings based on degree of hemolysis, into ''clear,'' ''slight,'' and ''gross/total'' samples, had a mean +/- standard devia tion of 0.150 +/- 0.072, 0.187 +/- 0.105, and 0.232 +/- 0.108, respect ively. The incorporation of sodium azide during the initial step of se rum dilution dramatically eliminated the background, giving a mean +/- standard deviation of 0.079 +/- 0.029, 0.076 +/- 0.022, and 0.081 +/- 0.029, respectively. The level of endogenous peroxidase activity, a p ossible factor for this nonspecific interference, was considerably ele vated in some of the swine sera. The clear, slight, and gross/total ca tegories had relative levels of 1%, 2%, and 51% peroxidase activity co mpared to the conjugate peroxidase activity of 100%. Whereas sodium az ide could be used only in sample predilution in the swine toxoplasmosi s peroxidase-conjugate test, in the bovine brucellosis alkaline phosph atase-conjugate card test it could be used in all wash cycles. Many br ucellosis card test results were visually uninterpretable because of s ignificant background color when the manufacturer's wash reagent was u sed. The substitution of a wash reagent containing sodium azide elimin ated background color, giving a visually unambiguous test.