AOAC Use-Dilution Method (UDM)

Disinfectant efficacy data generated using the Use-Dilution Method (UDM) of AOAC International is accepted by the U.S. Environmental Protection Agency (EPA) and Health Canada.

Separate UDM test protocols are designed to work with:
– Staphylococcus aureus (955.15)
– Salmonella enterica (955.14)
– Pseudomonas aeruginosa (964.02).Stainless steel penicyliders (Fig 1), used as representative hard nonporous inanimate surfaces, are contaminated with a test bacterial suspension and then exposed to the use-dilution of the test formulation. At the end of the contact time, each cylinder is separately placed in a tube with a recovery broth containing a neutralizer. The tubes are incubated and examined for turbidity as an indicator of bacterial growth.

Fig 1. Stainless steel penicylinders used as carriers in UDM

The main steps in this semi-quantitative method are summarized in the flow chart below.
For product registration, three production lots must be tested against each organism.

UDM has the following limitations:
– Semi-quantitative
– Operator-and technique-sensitivity may yield variable results
– Inoculum drying conditions not properly specified
– Ratio between carrier surface area and disinfectant volume too large
– Unsuitable for testing virucidal activity.
Efforts are now underway to replace UDM with quantitative carrier test protocols.