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ulations must be considered. We totally agree with his arguments. His constructive, detailed technical explanations should be tested thoroughly by the manufacturers of Ag/Cu ionization systems and taken into consideration in future experiments. Dr. Lin’s main scientific point is that the data we presented in our paper do not support the claim that Legionella developed resistance to silver. He assumes that Ag/Cu ionization in our hospital did not effectively control Legionella, even in the beginning. To support this, he gives the percentages of water samples from distal sites that were positive for Legionella (detection limit 1 cfu/L). However, we cannot draw valid conclusions from these values alone. Our statistical evaluation is based on Legionella counts (cfu/L) and not on “sample points positive for Legionella.” The results of the multiple regression analysis that we presented in our paper clearly revealed a decreased influence of Ag ions on Legionella counts during the 4-year study period. But even without performing a statistical evaluation the facts are as follows: in the first year after the Ag/ Cu ionization unit was installed, the percentage of samples positive for Legionella decreased from 100% to 55%, with an average Ag level of !10 mg/L; in the fourth year of Ag/Cu ionization, the percentage of samples positive remained at 75%, with an average Ag level of 30 mg/L. The methods and detection limits used in various reports concerning Ag/Cu ionization are not comparable. In order to facilitate comparison [2], we gave results as counts of Legionella cfu/L, not as positive distal sites per swab [3, 4]. We prefer a quantitative method in reporting effective disinfection, which is a common procedure in examining any disinfection method (see, e.g., [5]). Another question is whether there is any connection between the quantity of Legionella (cfu/L) in water distribution systems and the incidence of legionnaires disease. Indeed, there are many unresolved questions regarding the effectiveness of Ag/Cu ioniziation for control of Legionella in hospitals, including the influence of chemical water composition, temperature, and circulation on metal activity. Vigilance is necessary not only because Legionella may develop resistance to the activity of Ag and Cu, but also because of the question of protozoa inactivation by Ag and Cu ions. We believe that methods for disinfecting water distribution systems should be designed to control the growth of both Legionella and protozoa. But the investigations of Cassels et al. [6] showed that electrolytically-generated concentrations up to 80 mg/L Ag and 800 mg/L Cu did not inactivate Naegleria fowleri in vitro. Recently we reported that Hartmannella vermiformis survived at concentrations of 50 mg/L Ag and 500 mg/L Cu in vitro [7]. There was further discussion of the arguments about Ag/Cu ionization at a panel discussion on “Copper/Silver Water Ionization Systems: Pro and Con” at the Fifth International Conference on Legionella at Ulm, Germany, 26–29 September, 2000. Ute Rohr Institut für Hygiene und Mikrobiologie, Abteilung für Hygiene, Sozialund Umweltmedizin, Ruhr-Universität Bochum, Bochum, Germany

Limitations of Plasma Human Immunodeficiency Virus RNA Testing