Less than 1 CFU/Plate in ISO 5 Class - How is it Possible? : Pharmaceutical Guidelines

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Less than 1 CFU/Plate in ISO 5 Class - How is it Possible?

CFU is colony forming unit that may be anything like bacteria, fungus or yeasts. There is a limit of colony forming units in the classified area that must be followed by pharmaceutical products manufacturing facilities..
There are different cleanliness classes in pharmaceuticals. Class 100 or ISO 5 is the highest cleanliness class found on pharmaceutical industries and used in LAF and sterile filling area. this area is monitored for viable and non-viable particle counts. Some people have confusions about the limits of ISO 5 class and ask me for its clarifications.

All cleanliness classes have different standards for viable and non-viable particles but ISO 5 has <1 CFU per plate count for the viable count that makes the people confused because physically, the existence of less than 1 CFU is not passable. We can’t count colony forming units less than 1 (in decimals) then why all the regulatory guidelines have this specification for the ISO 5 class.

Less than 1 cfu/ plate
Suppose if you had exposed 5 plates and each had 1 CFU then the average will be 1 and according to the specification area fails in the viable count.

The average of all exposed plates should be less than 1. For example, if you had exposed 5 plates and all plates have the following number of colony forming units.

Plate
1
2
3
4
5
Count
1
0
1
1
0

Average of these counts is 3/5 = 0.6 CFU/plate and it is within the specification.

You can have more than 1 CFU in one or two plates. See the following results.

Plate
1
2
3
4
5
Count
2
0
2
0
0

The average of these counts is 4/5 = 0.8 CFU/plates and it also complies the specification.

Even you can have 4 CFUs in a single plate as below.

Plate
1
2
3
4
5
Count
0
0
4
0
0

The average of these counts is 4/5 = 0.8 CFU/plates and it also complies the specification.
None of the regulatory guidelines says that how much count could be there in a single plate. It means you should have 1 CFU less than the number of Petri plate exposed in class ISO 5 area to get average CFU less than 1 to comply with the specification. Therefore, the average of all exposed plates should be less than 1 CFU /plate but not in any individual plate.

In most of the cases when we expose the Petri plate or active air sampling, we don't everage the count of the plates sampled in the area. In such conditions, we should consider <1 CFU/plate as 0 CFU/plate because we can get <1 CFU only by averaging the results as described in the above examples.

However, it is not good to have a single CFU in ISO 5 area because the sterile filling is done here and it will contaminate the sterile product being manufactured in the area. Therefore, we should focus to get 0 CFU in such critical areas.





Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of Pharmaceutical Guidelines, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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6 comments: Post Yours! Read Comment Policy ▼

  1. Hi,
    Thanks for posting about this important topic. I personally disagree with the statements which implies averaging of data related to microbial contamination of the EM plates. This is simply for many reasons however most importantly because such bad practice of averaging may cover up some contamination during critical processing. for me, the meaning of "less than 1" in the regulatory guidance is the fact that we can not simply say "zero" as we may have some contamination which we are not aware of or not detected. I invite you to kindly study the topic further and amend the article accordingly. Thanks and kind regards, Mohamed

    ReplyDelete
    Replies
    1. Hi Mohamed,
      You have rights to have an opinion different from the article but I don't think regulatory guidelines have any hesitation in writing 0 cfu/plate.
      Furthermore, we are not monitoring environment directly but we check it in the terms of cfu/plate and we can write 0, if we don't find anything.
      If my interpretation for <1 cfu/plate is wrong then regulatory authorities must change it to 0 cfu/plate and every plate because anyone can interpret it wrongly like me.

      Delete
  2. Sir,we used to expose 1 plate at one location defined in sop. Then how this will work

    ReplyDelete
  3. Hi Ankur
    Thanks for the article. I have a doubt here, We have to report the final results at decimal points as specified in specification. To me 0.6, 0.8 are equal to 1 (based on rounding the figures). I agree if the specification says less than 1.0.

    ReplyDelete
    Replies
    1. We can't determine colony forming units in decimal, therefore we write <1.

      Delete


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