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Is the unit cfu/swab the same as cfu/cm2? If no, How do they differ?

Environmental Swabs

Best Answer bob1, 21 February 2019 - 01:22 PM

This is simple math. If you have swabbed an area of 100 cm2, then you need to divide your total counts by that area to get the cfu/cm2 unit: 607/100 = 6.7 cfu/cm2. It would also be correct to say 670 cfu/swab - however, if the area swabbed is not consistent, then this doesn't tell you anything useful as you have no way of normalizing the results. For example: if you had one swab where you swabbed an area of 25 cm2 and got a total of 400, and another where you swabbed 100 cm2 and also got 400 - which is more heavily contaminated? Without the area conversion you have no way of telling.

 

With regards the worksheets and having the different measures of reporting, this may be a result of standardized tests. For example, maybe you are always supposed to swab 100 cm2, so reporting per swab is a consistent measure, or it may be that this sort of thing is accounted for later in some other sheet/report that you do not see. Otherwise it could be a serious error in the methodology that your laboratory uses. If this is a clinical laboratory (say for environmental health and safety monitoring), then I would expect that they have these sorts of things worked out long ago and follow the appropriate standards under law. If this is the case, you can ask to see the relevant standards.  If you are in a research laboratory, research methodologies tend to be a bit less standardized and more loosely followed, so perhaps when the swabbing was first introduced, per swab was the same as per 100 cm2, but now this is not the case. However, in both cases you should be able to ask for clarification from a supervisor.

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#1 amaybea

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Posted 21 February 2019 - 12:32 PM

Please help: In reporting of Environmental Swabs testing, I am quite confused in my laboratory, because some worksheets write cfu/swab, and some cfu/cm2, for the same method of testing: I dry swab a surface area of about 100cm then vortex it into 10ml diluent for 30 secs, then pipette 1ml and dispense it into a petri dish, then pour about 10-15 ml VRBGA into the plate for EB or Enterobacteriacea enumeration. In the worksheet, the dilution is 1:10, so I multiply by 10 the total colony growth I count on the plate after incubation at 37C for 24hrs, e.g. 67 colonies counted x 10 = 670 cfu. My problem is, to know which is a correct unit, is it 670 cfu/swab or 670 cfu/cm2?

 

In our microbiology laboratory, no one knows the answer that is why I am writing in this forum. I tried to google but I did not find a definite answer. Your help is very much appreciated. Thank you in advance. Ruby



#2 bob1

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Posted 21 February 2019 - 01:22 PM   Best Answer

This is simple math. If you have swabbed an area of 100 cm2, then you need to divide your total counts by that area to get the cfu/cm2 unit: 607/100 = 6.7 cfu/cm2. It would also be correct to say 670 cfu/swab - however, if the area swabbed is not consistent, then this doesn't tell you anything useful as you have no way of normalizing the results. For example: if you had one swab where you swabbed an area of 25 cm2 and got a total of 400, and another where you swabbed 100 cm2 and also got 400 - which is more heavily contaminated? Without the area conversion you have no way of telling.

 

With regards the worksheets and having the different measures of reporting, this may be a result of standardized tests. For example, maybe you are always supposed to swab 100 cm2, so reporting per swab is a consistent measure, or it may be that this sort of thing is accounted for later in some other sheet/report that you do not see. Otherwise it could be a serious error in the methodology that your laboratory uses. If this is a clinical laboratory (say for environmental health and safety monitoring), then I would expect that they have these sorts of things worked out long ago and follow the appropriate standards under law. If this is the case, you can ask to see the relevant standards.  If you are in a research laboratory, research methodologies tend to be a bit less standardized and more loosely followed, so perhaps when the swabbing was first introduced, per swab was the same as per 100 cm2, but now this is not the case. However, in both cases you should be able to ask for clarification from a supervisor.



#3 amaybea

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Posted 21 February 2019 - 06:05 PM

Thank you so much, Bob, for that very helpful, prompt and detailed response. I highly appreciate your kindness and generosity to share your knowledge.






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