Model tests for the efficacy of disinfectants on surfaces. IV. Communication: dependence of test results on the amount of contamination and the kind of active substance.
In the assessment of efficacy of surface disinfectants, many influencing factors have to be taken into account. One essential item is whether the surface to be disinfected is clean or soiled. Among the feasible soilings, the blood is of particular consequences because it ads impediments to many disinfecting agents. This paper shows to what extent the impairment of the efficacy of typical active agents depends on the blood burden of the surfaces. Therefore, test surfaces (varnished plywood) were contaminated with 0.01 to 0.08 ml of coagulating blood per test area (3 cm2). The blood contained cells of Staphylococcus aureus as test germs. The disinfection was effected by immersing the test objects in the disinfecting solution for 5 seconds and mingling the adhering disinfecting solution (about 0.02 ml) with the coagulated blood on the test surface with a glass spatula for about 20 seconds. Subsequently, the test objects remained in a horizontal position at room conditions for 4 hours and then the numbers of surviving test germs were determined. The graphical representation of the results shows that the efficacy curves of formaldehyde and phenol lie very closely together, i.e. their effect is hardly impaired by the different blood burdens of the test areas. The efficacy curves of glutaraldehyde, peracetic acid, chloramine T, and quaternary ammonium compounds lie very far apart from each other. To achieve the same microbicidal effect (log N/N0 = -5) when the contaminating amount is raised from 10 microliters/3 cm2 to 80 microliters/3 cm2, the concentration of chloramine T has to be raised by a factor of 5.4, peracetic acid by a factor of 9, glutaraldehyde by a factor of 24, quaternary ammonium compound even by a factor of 67. Ethanol and sodium hypochlorite showed a divergent behaviour. For ethanol, the efficacy diminution produced by increasing the contamination amount by a factor of 4 can be compensated by raising the concentration from 50% to about 70%. But again and again, there were test objects on which the number of germs able to reproduce had only been lowered by a factor of about 10(-3). At the highest contamination of 80 microliters/3 cm2, even 95% ethanol proved to be completely insufficient. With sodium hypochlorite even at the lowest contamination of 10 microliters/3 cm2, a microbicidal effect of only about 10(-5) was obtained. With increasing contamination, the highest achievable microbicidal effect clearly decreased. It is remarkable that the microbicidal effect of this active agent decreased with increasing concentrations. The results show how important it is in testing the efficacy of disinfecting agents to exactly lay down the amount of contaminating substances. To find out how safely an agent works under harder circumstances, the dependence of the microbicidal effect from the amount of contaminating substances per test area has to be determined.