D-value determinations are an inappropriate measure of disinfecting activity of common contact lens disinfecting solutions

Determination of a D value for specific test organisms is a component of the efficacy evaluation of new contact lens disinfecting solutions. This parameter is commonly defined as the time required for the number of surviving microorganisms to decrease 1 logarithmic unit. The assumption made in establishing a D value is that the rate of kill exhibits first-order kinetics under the specified conditions. Such exponential kill rates are seen with thermal contact lens disinfection systems. A comparison of the death rate kinetics for a variety of chemical contact lens disinfecting solutions was undertaken to ascertain the suitability of D-value determination for these chemical disinfectants. The active agents of these different solutions included hydrogen peroxide, thimerosal, chlorhexidine, tris(2-hydroxyethyl)tallow ammonium chloride, thimerosal, polyaminopropyl biguanide, and polyquaternium-1. The solutions were challenged with 106 CFU of either Pseudomonas aeruginosa, Serratia marcescens, or Staphylococcus hominis per ml, and survival rate was determined. This study clearly demonstrates the nonlinear nature of the inactivation curves for most contact lens chemical disinfecting solutions for the challenge organisms. D-value determination is, therefore, an inappropriate method of reporting the biocidal activity of these solutions. Therefore, when the log1o numbers of the survivors were plotted against time, the relationship was described by a straight line. The D value is broadly defined as the time required for the number of viable bacteria to decrease 1 logarithmic unit (or the negative reciprocal of this slope). This D value is then used as a predictor for responses beyond the data to estimate the time required for disinfection (10-' CFU/ml) or sterilization (10-6 CFU/ml). The underlying assumption when utilizing this measure is that the relationship between the log1o number of survivors and time is linear This assumption holds true, in general, for thermal disinfection of contact lenses (9, 10). However, the application. of D-value determination is gaining acceptance in the pharmaceutical industry All three of these methods will provide similar values for rate of kill when the kinetics of kill are first order. If the relationship between log1o number of survivors and time is not linear, however, very different measurements will result. These differences will have a direct effect on the final marketed product. Current guidelines set the recommended soak time for contact lens disinfecting solutions as equivalent to 9 D values (32). Although a straight line has only a single slope, a curvilinear line has many. Accordingly, the three methods for D-value determination of contact lens disinfecting solutions have the potential for disparate and inaccurate representation of disinfecting efficacy in cases of nonlinear kinetics. The present study evaluated several different, currently marketed contact lens disinfecting solutions. They were tested for disinfecting efficacy against Staphylococcus hominis, Pseudomonas aeruginosa, and Serratia marcescens, organisms representative of ocular bacterial pathogens. Previous reports provided D values for several of these solutions without consideration of linearity (21-24), a necessary precondition. In the present study, the inactivation curves 202