Gamma counters for radioimmuno and immunoradiometric assay

A considerable demand can be observed for gamma counters that can be used in laboratories of medical clinics to carry out radioimmunoassay (RIA) and immunoradiometric assay (IRMA). It the result of growing use of such assays in contemporary nuclear medicine. Large analytical laboratories use multi well gamma counters and manual insertion of samples into detector wells, or counters with one or two wells and automatic insertion of the samples. For medium size, or small size laboratories, gamma counters with single or a few measuring channels and manual insertion of the samples seems adequate. Two types of gamma counters were developed for small and medium size laboratories: single well and four well gamma counter with manual insertion of analyzed samples into detector well. The way hardware and software of the counters is solved is the subject of the presentation. Well Nal(TI) scintillators are used for detection of 125I radiation followed by single channel analyzer 15-80 keV window width. Photomultiplier gain is automatically checked up before start of each of measurement of count rate, and if needed the gain is corrected. Calibration process, including measurement of control samples, and then measurement of unknown samples is fully programmed in advance and is carried out under the control of the software. It includes number of reference samples for construction of calibration curve and their concentration, number of control samples, number of repetition of the samples with the same concentration, counting time. The calibration curve for RIA assay has the form lin(A/Ao) versus log(CONC) whereas for the IRMA assay the form log(A/T) versus log(CONC), where A- count rate of successive sample, Ao-count rate of sample with zero concentration, T - count rate of TOTAL sample, CONC-antigen concentration. The results of calibration and analysis of unknown samples, together with measuring parameters, date and time of the analysis, are printed with an external printer, producing thus a hardcopy document of the analyses made. If needed, the measuring results can also be transmitted to an external computer. Instructions displayed for the user facilitate operation of the counter. Additional software control functions and internal pulse counter allow to check correct operation of the gamma counters. The internal pulse counter can even be used to investigate new ideas. Detection efficiency of 125I is 75%. Long term instability is lower than ± 1%. (author)