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) and
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:
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 activityEfforts are now underway to replace UDM with quantitative carrier test protocols.