Labeled immunoassays use a labeled reactant like an enzyme or radioactive substance to indirectly detect the presence of small or low concentration antigens and antibodies. There are two major formats - competitive assays where labeled and unlabeled analyte compete for antibody binding sites, and noncompetitive assays like ELISA where the amount of label detected is directly proportional to the amount of analyte present. Enzyme immunoassays are now more commonly used than radioimmunoassays due to their simplicity, sensitivity, specificity and lack of health hazards from radioactivity. Rapid membrane-based cassette and immunochromatography assays can provide qualitative results without complex instrumentation.