In theory, one QD coated with multiple streptavidin molecules can accept multiple biotinylated antibodies. Thus, to maximize the illumination efficiency (one antibody per QD), strategy 1 or 2 could provide excessive QDs to those cell-bound antibodies after the removal of excessive free antibodies in the reaction system or could reduce the numbers of antibodies binding to one QD before labeling target cells with antibody-QD conjugates, respectively. Our results indicated that, under the manufacturer's suggested QD working concentration (20 nM), both strategies achieved a similar maximal S/N ratio of 17 for the labeling of C. parvum. Strategy 1 further showed a slight but gradual decrease in the S/N ratio to 16.5, 15.1, 10.8, and 7.5 under reduced QD concentration at 10×, 100×, 1,000×, and 10,000× dilution of the working concentration, respectively. In contrast, strategy 2 exhibited a rapid decrease in the S/N ratio to 15.0, 6.3, 6.0, and 3.8 under reduced QD concentrations at 3×, 9×, 27×, and 160× dilution of the suggested concentration, respectively. Further 3- to 10-fold increases in the QD working concentration with both strategies also caused an increase in background noise and a decrease in the S/N ratio to 9.2 to 12.1. Thus, a 3×- to 10-diluted QD working concentration was optimal for both strategies to maximize the S/N ratio.