What is an Inhibitory Residue Test?

Q. What is an inhibitory residue test?

A. The inhibitory residue test (IRT) is a test of suitability of glassware for microbiology testing.  The test is described in Standard Methods SM 9020B3a (Editions 18, 19) or SM 9020B4a (Editions 20, 21).  It is similarly described by the US Environmental Protection Agency in SOP Number: QC-03-07 and EPA’s “Manual for the Certification of Laboratories Analyzing Drinking Water” (EPA 815-R-05-004, January 2005).

Note that Standard Methods says to do the test “as necessary.” This will be determined by your specific regulatory needs and objectives.

Regarding what they certify, the IRT tests the detergents used for washing glassware in the microbiology section of a lab, or in a water/environmental lab, to make sure that they do not have bacteriostatic or inhibitory qualities that may affect the microbiological test that you perform.

Secondly, it also “checks” your washing procedure to make sure that if there are any inhibitory/bacteriostatic substances present in your detergent, the cleaning process that you employ washes out any of these inhibitory substances.

Labs that are certified for drinking water microbiology have to perform the inhibitory residue test as well as the distilled water suitability test annually.

Alconox, Inc. is proud to annually test each detergent brand through an independent laboratory. The certificates, containing the current and previous 3 years data, are located on the Alconox.com IRT page.  These tested detergent brands include:

To get a lot number specific inhibitory residue test to comply with accreditation guidelines requiring lot specific or annual testing, a “master lot number” is required.  These can be located on your detergent lot number’s specific COA.

These detergent lot numbers are a “sub lot” of a specific master lot. Once the master lot number is identified on the COA form, the inhibitory residue test data can be downloaded. Master lot numbers change such that each lot can be traced to a Master lot that was tested within one year. Inhibitory residue tests are performed for each year’s worth of sub lot numbers.

Testing is typically completed in the 2nd quarter of each year. Detergent purchased from lots made in the second half of the year will have the same year testing available. Lots manufactured in the first half of the year will refer back to prior year testing done within the past 12 months.

We are always happy to discuss further.

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