Optimization of the fluorescence in situ hybridization (FISH) technique for high detection efficiency of very small proportions of target interphase nuclei

Using commercially available fluorochrome‐labeled probes specific for chromosomes X, Y, 13, 18, and 21, we optimized the technical protocols for fluorescence in situ hybridization (FISH) so that the highest sensitivity and specificity were achieved. Also, we compared the optical properties of different types of fluorescent labels in an effort to develop the most efficient FISH protocol, including the determination of which types of labels are the easiest to count accurately. The lymphocytes were purified from blood of normal male and female newborns, normal male and female adults, and a trisomy 21 male adult. Male and female lymphocytes were mixed in five different combinations. For each combination, the male lymphocytes either from newborns or from adults were diluted with female lymphocytes in seven different proportions. For each of these 35 different cell mixtures, 100 000 nuclei were analyzed and scored in a blind fashion. Among the different fluorochrome‐labeled probes, the highest sensitivity and specificity were achieved when SpectrumAqua CEP‐Y/SpectrumOrange CEP X probe mixture, SpectrumAqua CEP‐18, SpectrumOrange LSI‐13, and SpectrumOrange LSI‐21 were hybridized. The hybridization sensitivity and specificity were higher than 99% for the identification of chromosomes X, Y, 13, and 18, and higher than 98% for the detection of trisomy 21. The proportion of false‐positive signals was under 0.005% for XY detection and lower than 0.14% for autosome detection. With these high hybridization sensitivities and specificities, the optimized FISH protocol developed in our laboratory has the potential to detect very rare events, e.g., when the proportion of cells being sought is lower than 0.01%. In other words, our protocol allows the specific detection of one male cell sunken among 10 000 female cells.