9 replies to this topic

[SheenaQA&BRCGS]

We do micro testing on spices using rapid total aerobic and yeast and mould rapid plates. Once a year we do an annual proficiency test to compare our results with an external laboratory. Their method is different therefore the results are not the same. They used agar plates and the numbers aren't the same. Our result was 10,000 (3 times dilution) and theirs was 540.

Can anyone elaborate on this? How can I compare the results?

Thanks in advance

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[SHQuality]

How do the methods differ. Do you both use a standard?

[Scampi]

I would ask the lab if they can use the same method OR you use the plates they use and compare

 

Either way---you should try and compare apples to apples annually, and right now your comparing apples to oranges

 

Are you certain they only did 3 dilutions?  I used to do 4

[SheenaQA&BRCGS]

I would ask the lab if they can use the same method OR you use the plates they use and compare

Either way---you should try and compare apples to apples annually, and right now your comparing apples to oranges

Are you certain they only did 3 dilutions? I used to do 4

They dilute only once where we dilute 3 times for bacteria. Perhaps we could try diluting once to compare to their results

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[Scampi]

that's a good start!

[Brothbro]

Yes as others have mentioned you really need to compare the same methods. It sounds like you're using 3M Petrifilm, correct? The instructions call to adjust the pH of the sample within a certain range before plating. I have seen many facilities neglect that step. But more importantly, you should both be testing the same method. As far as I understand, proficiency testing is more about gauging your staff's proficiency and not the method's accuracy. That's why it's best for both you and the lab to use the same method, sample matrix, dilution factors, and counting rules. The only variable here should be how well your staff are handling themselves in prep/plating.

 

If you're wondering whether your chosen rapid method is suitable for your testing purposes, that's kind of another testing scheme. I'm not experienced in it, but there is a way to determine if your rapid method is suitable for testing your product. Oftentimes this type of testing is outsourced to a 3rd party lab entirely.

[Charles.C]

We do micro testing on spices using rapid total aerobic and yeast and mould rapid plates. Once a year we do an annual proficiency test to compare our results with an external laboratory. Their method is different therefore the results are not the same. They used agar plates and the numbers aren't the same. Our result was 10,000 (3 times dilution) and theirs was 540.

Can anyone elaborate on this? How can I compare the results?

Thanks in advance

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Hi Sheena,

 

The only conclusion is that one or both results is/are incorrect.

 

It's a common problem in micro testing. The only reliable solution is for both ends to use an approved  (eg AOAC) method however many labs will not agree to this, especially IMEX at the customer's end. The result can be conflict if a rejection is involved.

 

@Brothbro - Sorry but your "as far as I understand" is not in accordance with the usual interpretation of "Proficiency". Can refer to previous SQF threads here where the original text of Standard IIRC is beautifully unclear.

 

PS - if yr external lab is ISO17025 accredited and using an approved procedure, then the odds are probably in their favour. You might consider asking the supplier of rapid plates as to suitability and mention yr problem. IMEX this will generate a fairly rapid response although unless you have a resident qualified microbiologist, communication may be difficult.

[SHQuality]

We do micro testing on spices using rapid total aerobic and yeast and mould rapid plates. Once a year we do an annual proficiency test to compare our results with an external laboratory. Their method is different therefore the results are not the same. They used agar plates and the numbers aren't the same. Our result was 10,000 (3 times dilution) and theirs was 540.

Can anyone elaborate on this? How can I compare the results?

You can't compare these results.

 

Your result is over 60x higher than that of the external lab. Either you are dealing with a contamination during your testing procedure or one of the methods is not applied correctly.

[SheenaQA&BRCGS]

You can't compare these results.

Your result is over 60x higher than that of the external lab. Either you are dealing with a contamination during your testing procedure or one of the methods is not applied correctly.

I believe both our method and the labs methods are valid. MFHPB-33.2015 vs MFHPB-18. 10,000 is much below our specification for total aerobic plate count (we are testing spices, just to answer your contamination concern) I now understand the method they are using and they're only diluting once. Definitely can't compare these results so I think an option would be to use their method for proficiency or find a lab that uses our method

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[SheenaQA&BRCGS]

Hi Sheena,

The only conclusion is that one or both results is/are incorrect.

It's a common problem in micro testing. The only reliable solution is for both ends to use an approved (eg AOAC) method however many labs will not agree to this, especially IMEX at the customer's end. The result can be conflict if a rejection is involved.

@Brothbro - Sorry but your "as far as I understand" is not in accordance with the usual interpretation of "Proficiency". Can refer to previous SQF threads here where the original text of Standard IIRC is beautifully unclear.

PS - if yr external lab is ISO17025 accredited and using an approved procedure, then the odds are probably in their favour. You might consider asking the supplier of rapid plates as to suitability and mention yr problem. IMEX this will generate a fairly rapid response although unless you have a resident qualified microbiologist, communication may be difficult.

Thanks, I will look into the suitability of 3M rapid plates

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