Remember you need to get to the root cause of each Presumptive positives. Seek and destroy. Each presumptive positives need to be treed as positive. You need to do corrective action on each and everyone.
Sorry but I totally disagree.
IMO Each presumptive needs to be confirmed. Negative results imply no "immediate" concern.
I recommend the OP to urgently seek a professional microbiologist.
PS - One can find opposite opinions to the above in the Literature. Offhand i suggest these are "alarmist".
For example -
There is no need to further analyze samples after a positive test result
All commercially available rapid methods are screening tests and require cultural confirmation after a “presumptive positive” test result. Confirmation requires streaking the enriched test portion to selective agars in order to isolate a typical colony based on morphological and biochemical characteristics. This is especially true when testing end-products or foodstuffs. For environmental samples, cultural confirmation is seldom performed as added sanitation practices are often put in place [!!!]. There are several options for selective agar plates, with the most common ones found in the USDA Microbiology Laboratory Guidebook (MLG), FDA Bacteriological Analytical Manual (BAM), or ISO reference guides (see Table 1).
Positive pathogen detections
It is a legal requirement for licensed businesses to notify the Food Authority about positive pathogen detections. Notification should be given by phone within 24 hours of receiving the laboratory result, and in writing within seven days. Once the laboratory advises that a presumptive positive result has been recorded, there is usually a 24-hour delay before the result is conclusive.
Businesses should use this time to plan a product recall or withdrawal, in the event that a positive result is confirmed.
NSW - presumptive_Listeria_positive.pdf 288.99KB
from BAM -
Alternate Screening Methodologies
The following alternative screening methodologies may be used to screen samples for the presence of Listeria. Follow the manufacturers' package insert making certain they have not deviated from the approved versions of the AOAC INTERNATIONAL Official Methods Manual protocols (Section F1). The kits are only approved for the specified food matrices, claimed in the OMA method, which vary from kit to kit. For other food matrices that are not validated a matrix extension validation is necessary. Negative results obtained with the products are considered definitive and no further testing is required. Presumptive positive results with these rapid screening methods must be confirmed by streaking to the selective agars and confirming isolates to the species level by the procedures described in sections G-I.
PPS - one caveat to my opinion could be perhaps where published data exists that, say, 95% of the presumptive positives typically found for environmental samples do confirm positive (might also relate to nature of business). This would depend on the degree of specificity of the particular screening test. I have as yet not seen any such data for Listeria kits but perhaps it exists (?).
P3S - I have previous experience of a rapid Salmonella test kit for which, over a period of approx 2 years, every positive presumptive it found failed to confirm. Kit was not expensive but was simply insufficiently selective for the type of product I was handling.
Out of curiosity, what do FSIS recommend in their encyclopedic testing programs for Listeria Control ? Anyone ?
added - I guess this may answer my query -
Presumptive positive results must be confirmed according to the confirmation procedure outlined in the USDA-FSIS MLG 8.09 (Isolation and Identification of Listeria monocytogenes from red meat, poultry, egg products, and environmental samples) or the FDA’s Bacteriological Analytical Manual (or BAM Chapter 10: Detection and Enumeration of Listeria monocytogenes in Foods).