How alternative are immunoassay systems employing non-radioisotopic labels? A comparative appraisal of their main analytical characteristics.

The immunoassay is one of the most sensitive and reliable analytical techniques available in the clinical laboratory. The original label for immunoassays was radioisotopes, and these methods, radioimmunoassay (RIA) and immunoradiometric assay (IRMA) are still the reference methods, because of invulnerability of the radioactive emission with respect to environmental interference. Labels other than radioisotopes have been tested for use in immunoassay to improve the sensitivity and reliability and to avoid some of the disadvantages of radioisotopic techniques. New labels have continued to be developed (Horseradish peroxidase-HPR-, pyrophosphatase, luciferases, pyrodopirazines, europium cryptates, porphirins, phosphors) and new label detection methods have been set up (e.g. chemiluminescence assay, thermometric assay, NADP+ and FADP- based coupled assay). New immunoassay strategies such as simultaneous multianalyte automated test have been developed and the reliability of the assays has in some cases caused division among researchers about the choice between the radioisotopic immunoassay or the non-radioisotopic immunoassay, as considerable effort and investment had been devoted to the search for more sensitive and practicable tests than the classic RIA-IRMA methods. The evolution of immunoassays (Monoclonal Antibodies, non-radioactive tracers, automation) has produced systems which allow a large number of laboratories to determine a great number of analytes with very good practicability. The availability of fully automated systems has generated the opinion that analytical performance of immunoassays can be considered similar to that of many traditional parameters of clinical chemistry. This conclusion seems however too optimistic, in fact data collected from interlaboratory studies demonstrate that problems concerning the analytical reliability of the measurements still remain not completely solved. In the authors' opinion, this opposition between immunological assay based on isotopic or non-isotopic labels is misleading, because each assay (whether it uses isotopic, enzymatic, fluorimetric or luminescent labels) has its own analytical characteristics and performance. For this reason the term "alternative", used to indicate all non-isotopic assays as a unique class of tests, should be abandoned. From a theoretical point of view the choice should not be between isotopic and non isotopic techniques. For each analyte to be tested, it is advisable to use the immunological assay that suits the requirements of the laboratory, irrespective of type of label. From a practical point of view, the choice should be based on the analytical performance and on the characteristics of each assay, on its cost and the type of instrumentation available in the laboratory, and on the experience and the knowledge of the laboratory personnel.