Effect of Diluent on Molecular Iodine Release From On-Site Diluted Povidone-Iodine

The global expansion of antibiotic-resistant organisms has led to a renewed interest in antiseptic principles of infection prevention. On-site dilutions of povidone-iodine (PI) with normal saline have increased substantially, but little information exists regarding the concentration of the singular active biocidal agent, i.e., molecular (free) iodine, in these formulations. The purpose of this study was to analyze the differences in the amount of molecular iodine released from on-site dilutions of five different commercially available 10% PI products and three different dilution ratios in normal saline and in sterile water.

Potentiometric analysis of commercial solutions used an iodine-selective electrode with an Ag/AgCl reference and platinum working electrodes. Calibration was conducted with 0.345 g/L aqueous iodine solutions. Samples were diluted with relevant solvents. On electrode equilibration, electrochemical potentials were recorded. There were six different measurements taken from each dilution rate, and free iodine levels were measured. We used two-way analyses of variance to determine the significance of dilution rate and solution type.

Increased dilutions of 0.35, 0.22, and 0.10% PI in normal saline resulted in significant increases in the values of molecular iodine (P < 0.001). There was also a significant effect from type of diluent, with sterile water producing higher measurements of molecular iodine versus normal saline in all dilution ratios (P < 0.001).

In chemistry and microbiology literature, molecular iodine is the only iodine species in iodophor solutions with a proven correlation between equilibrium concentrations and bactericidal activity. By using an on-site dilution of 0.10% povidone-iodine in normal saline or sterile water, we confirmed a significant, paradoxical increase in the concentration of the bactericidal molecular iodine. This supports a 0.10% on-site dilution of PI in normal saline or sterile water to achieve a larger therapeutic index and enhanced antimicrobial rate of kill.