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Questioning the reliability of all negative pathogen test results


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mer1in

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Posted 27 September 2020 - 07:08 AM

Hello..this is merlin, I work for an aviation food lab in kuwait. We get our PT samples from IFM & this time our results are low for non pathogens & pathogens all 3 samples are negative. Have anyone else used their services, just wondering if its possible to get all negative results? samples 20P6NP,20P6FP



Charles.C

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Posted 27 September 2020 - 01:04 PM

Hello..this is merlin, I work for an aviation food lab in kuwait. We get our PT samples from IFM & this time our results are low for non pathogens & pathogens all 3 samples are negative. Have anyone else used their services, just wondering if its possible to get all negative results? samples 20P6NP,20P6FP

 

Context is unknown but any combination is possible.


Edited by Charles.C, 27 September 2020 - 02:46 PM.
edied

Kind Regards,

 

Charles.C


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mer1in

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Posted 28 September 2020 - 09:44 AM

Thank you, was wondering since we have never got all negative test samples.



LesleySR

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Posted 28 September 2020 - 12:25 PM

Probably a question best asked of your supplier?...

 

A good supplier of PT material should be open to providing advice if you failed to detect a microorganism at low levels & would give information on methodology if  your standard pathogen test method fails to recover a positive result on a sample..

 

Very often a company will use a specific test method, that is applicable for their product group - for example meat products or chocolate - because they know it is reliable in that particular product matrix but the sample sent to you by the PT supplier is usually dried, so the method you use may not be 100% accurate for that specific organism or for that product matrix?.

 

If your results are incorrect - you should be able to explain why your test failed to detect the pathogen in the sample.



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Charles.C

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Posted 28 September 2020 - 07:07 PM

Probably a question best asked of your supplier?...

 

A good supplier of PT material should be open to providing advice if you failed to detect a microorganism at low levels & would give information on methodology if  your standard pathogen test method fails to recover a positive result on a sample..

 

Very often a company will use a specific test method, that is applicable for their product group - for example meat products or chocolate - because they know it is reliable in that particular product matrix but the sample sent to you by the PT supplier is usually dried, so the method you use may not be 100% accurate for that specific organism or for that product matrix?.

 

If your results are incorrect - you should be able to explain why your test failed to detect the pathogen in the sample.

 

Hi Lesley,

 

Some factors -

 

(1) Product unknown. I assumed solid.

(2) 3 assumed different random samplings involved.

(3) Same(?) source lot(s) unknown.

(4) Specific pathogen(s?) unknown.

(5) Test Procedure(s?) unknown.

(6) Level(s) of contamination unknown.

 

For low levels of contamination, (6) is a well-known and ever-present cause of significant sampling variations, even from the same lot.. More precisely, the probability of sample size (n=3) detecting instances of a pathogen like Salmonella in a given Lot is poor.

However, one reliable positive result does imply that some level of contamination exists, even though resampling may not detect it.


Kind Regards,

 

Charles.C


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mer1in

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Posted 26 October 2020 - 08:20 AM

Hi Lesley,
 
Some factors -
 
(1) Product unknown. I assumed solid.
(2) 3 assumed different random samplings involved.
(3) Same(?) source lot(s) unknown.
(4) Specific pathogen(s?) unknown.
(5) Test Procedure(s?) unknown.
(6) Level(s) of contamination unknown.
 
For low levels of contamination, (6) is a well-known and ever-present cause of significant sampling variations, even from the same lot.. More precisely, the probability of sample size (n=3) detecting instances of a pathogen like Salmonella in a given Lot is poor.
However, one reliable positive result does imply that some level of contamination exists, even though resampling may not detect it.


Thank you for your reply. Here are the details,

Product - freeze dried milk powder
They give us 3 packs with different levels of detection for the same set of micro organisms
Pathogens for detection - salmonella, listeria, staph.
we use automated testing - Tempo & vidas.
levels of contamination - of course unknown, till we hear back from the PT provider with our results.


mer1in

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Posted 26 October 2020 - 08:23 AM

Probably a question best asked of your supplier?...
 
A good supplier of PT material should be open to providing advice if you failed to detect a microorganism at low levels & would give information on methodology if  your standard pathogen test method fails to recover a positive result on a sample..
 
Very often a company will use a specific test method, that is applicable for their product group - for example meat products or chocolate - because they know it is reliable in that particular product matrix but the sample sent to you by the PT supplier is usually dried, so the method you use may not be 100% accurate for that specific organism or for that product matrix?.
 
If your results are incorrect - you should be able to explain why your test failed to detect the pathogen in the sample.


Dear Lesley,

Thanks, makes sense. I was just wondering if there was anyone who has taken the similar tests with the same outcome. The PT provider will get back with our results only in a few months.


Charles.C

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Posted 26 October 2020 - 09:22 AM

Thank you for your reply. Here are the details,

Product - freeze dried milk powder
They give us 3 packs with different levels of detection for the same set of micro organisms
Pathogens for detection - salmonella, listeria, staph.
we use automated testing - Tempo & vidas.
levels of contamination - of course unknown, till we hear back from the PT provider with our results.

Hi Merlin,

 

Thks yr response but it's rather confusing.

 

What does PT mean ? and IFM ?

 

^^^(red) - What does this mean ? eg Are you talking about pre-standardized samples or ?

 

You get the results in months ? Should be less than 1 week if all "negative" ?

-----------------------------------------------------------------------------------------------

I deduce you mean that for each of the 3 samples,

 

(i) Salmonella = Not detected in X grams

(ii) Staph.aureus (Coagulase Positive) = Not detected in Y grams

(iii) L.monocytogenes = Not detected in Z grams.

(X, Y, Z will depend on the Lab. Procedure)

 

I would have hoped that the above (i-iii) was the usual result for your samples ?


Kind Regards,

 

Charles.C


mer1in

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Posted 27 October 2020 - 10:07 AM

Hi Merlin,
 
Thks yr response but it's rather confusing.
 
What does PT mean ? and IFM ?
 
^^^(red) - What does this mean ? eg Are you talking about pre-standardized samples or ?
 
You get the results in months ? Should be less than 1 week if all "negative" ?
-----------------------------------------------------------------------------------------------
I deduce you mean that for each of the 3 samples,
 
(i) Salmonella = Not detected in X grams
(ii) Staph.aureus (Coagulase Positive) = Not detected in Y grams
(iii) L.monocytogenes = Not detected in Z grams.
(X, Y, Z will depend on the Lab. Procedure)
 
I would have hoped that the above (i-iii) was the usual result for your samples ?


PT - Proficiency Testing
IFM is the service provider (PT samples provider)
It's 3 sample packets A,B,C for the detection & enumeration of staph, listeria & salmonella. We need to test each packet as a separate sample for all the pathogens mentioned & submit our results to IFM, then usually in a month or 2 they will send us a report with the how accurate our results were, compared to theirs.
Anyway we will get our results in a week, will let you know the outcome.


Charles.C

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Posted 30 October 2020 - 06:54 PM

PT - Proficiency Testing
IFM is the service provider (PT samples provider)
It's 3 sample packets A,B,C for the detection & enumeration of staph, listeria & salmonella. We need to test each packet as a separate sample for all the pathogens mentioned & submit our results to IFM, then usually in a month or 2 they will send us a report with the how accurate our results were, compared to theirs.
Anyway we will get our results in a week, will let you know the outcome.

Hi mer1en,

 

Thks for clarification. Now I have some idea as to what yr OP was talking about. :smile:

 

Does yr history of testing give you any idea as to the typical "seeding" pattern of samples for yr testing ?.

 

Offhand it does seem rather unlikely that all  three submitted samples would have zero seeded pathogens unless this is a control to elucidate yr ability to generate false positives.

 

Some large-scale international ring tests of this type have in the past been done for Salmonella. The results tended to demonstrate the failure of many labs to be competent to detect Salmonella. However analytical routes/reagents to evaluate Salmonella have expanded significantly in the last 1-2 decades.


Kind Regards,

 

Charles.C





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