Alternatives to radioimmunoassay: labels and methods.

We review the following labels used as substitutes for radioisotopes in immunoassay systems: bacteriophages, chemiluminescence precursors, fluorochromes, fluorogens, fluorescence quenchers, enzymes, coenzymes, inhibitors, substrates, various particulates, metal atoms, and stable free radicals. New methods for performing immunoassays with these labels are described where appropriate. Methods that require no separation steps and offer special promise for easy automation are noted.

A sensitive manual enzyme immunoassay for thyroxine.

We describe an immunoassay for thyroxine in serum. In the assay specific antibody covalently bonded to latex particles is used, along with horseradish peroxidase as the label, and o-phenylenediamine as the chromogen. The flexible protocol is designed for manual execution. Performance is similar to that of the highest-sensitivity thyroxine radioimmunoassays. Results correlate well with radioimmunoassay (r = 0.99, slope = 0.93, y-intercept = 2.4 microgram/liter for 201 samples) and an automated enzyme immunoassay (r = 0.97, slope = 0.99, y-intercept = 4.7 coefficients of variation are less than 7.2% over the entire useful range of the assay (20--240 microgram/liter). The limit of detection is less than 94 pg/tube at 20 microgram/liter. Only D-thyroxine is known to interfere with serum assays. This assay has no discernible protein effect from 40 to 80 g of protein per liter, unlike many thyroxine radioimmunoassays. Serum preservatives known to be peroxidase inhibitors do not adversely affect assay performance because of the 56-fold dilution in the final assay mixture. Hemolyzed serum and EDTA-treated plasmas are unsuitable for this assay.

Solid-phase enzyme immunoassay for herpes simplex virus.

An enzyme immunoassay was used for the rapid detection of herpes simplex virus type 1 (HSV-1). The test utilized specific antibodies to HSV-1 attached to a solid phase. A laboratory preparation of HSV-1 was detectable in amounts of greater than or equal to 50 50% tissue culture-infective doses by this method. Tests performed with clinical samples indicated a specificity of 95%. The sensitivity appeared to depend on the length of time the samples were stored. When samples were stored frozen for three to six months, the sensitivity was 46% of that of tissue culture isolations performed when the samples were first collected. The sensitivity was 89% in comparison with attempts at tissue culture reisolation with the same samples at the time of enzyme immunoassay. The immunoassay also detected HSV in seven of 10 positive samples that were stored frozen for no more than two weeks before testing.