What is an Inhibitory Residue Test?
The inhibitory residue test is a test of suitability of glassware for microbiology testing. It was important when labs were using phosphate detergents, but is less so now that use of phosphate is being phased out. The test is described in SM 9020B3a (Editions 18, 19) or SM 9020B4a (Editions 20, 21).
Standard Methods says to do the test "as necessary." You might ask your regulator how they interpret that.
First and foremost, it tests the detergents/soaps used for washing glassware in the microbiology section of a lab to make sure that they do not have bacteriostatic or inhibitory qualities that may affect the microbiological test that you perform (such as multiple tube fermentation technique...). Secondly, it also "checks" your washing procedure to make sure that if there are any inhibitory/bacteriostatic substances present in your soap, 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. Another source of information about these tests is in EPA's "Manual for the Certification of Laboratories Analyzing Drinking Water". The most current is the 5th Edition published in 2005.