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Wed, 10 May 2006 07:53:43 +1200
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Kate
Thank you for your advice. I now realise that there is a third way of
interpreting the results:

Example C

Count/ml    480,000           170,000           28,000
Log (2 sig figures)   5.68          5.23        4.45
Log reduction        -        0.45        1.23
Log reduction (rounded)       0           1

Isn't it interesting what you can do with figures! (cf example 2)
I thank all of the forum members that have contributed to this question.
Your assistance is very much appreciated.
Brendan Lincoln



                                                                       
             "Ambrus, Kate"                                            
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             <[log in to unmask]         Re: [PMFLIST] Preservative testing
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             09/05/06 04:43                                            
                                                                       
                                                                       
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I would fail the first product, because 0.45 technically does not round off
to 1. It rounds off to 0.5 only if you are rounding to one decimal place.
But since you are rounding off to zero decimal places, it rounds off to 0
(if in doubt - check in EXCEL). I am not a mathematician, but I think it
has something to do with the number of rounding off steps you are allowed
to make (one only). If you had obtained a log drop of 0.5 to begin with,
then I would round it up to 1 and pass it (although with some
reservations).

The results in B seem misleading. We always calculate log-drops on the raw
data, and then round off the final results. That is we do not round off the
plate counts - only the resulting log-drops. Your actual log-drop was 1.23,
which does not round up to 2 - as Example 2 would seem to suggest.

Kate S. Ambrus
Manager, Eye Care R&D Microbiology
Advanced Medical Optics
[log in to unmask]
(714) 247-8318

-----Original Message-----
From: The Pharmaceutical Microbiology Forum Email List
[mailto:[log in to unmask]] On Behalf Of [log in to unmask]
Sent: Sunday, May 07, 2006 7:15 PM
To: [log in to unmask]
Subject: [PMFLIST] Preservative testing

Can members of the forum offer their opinion on how to evaluate a
preservative efficacy result.

Given that the Ph.Eur states in the General Notices for Limits:

"In determining compliance with a numerical limit the calculated result of
a test or assay is first rounded to the number of significant figures
stated, unless otherwise prescribed.
The last figure is increased by one when the part rejected is equal or
exceeds one half unit, whereas it is not modified when the part rejected is
less than one half unit."

Consider the following example being a result for an oral product
challenged with A.niger where the requirement is for a log reduction in the
viable count of log 1 at 14 days with no increase thereafter:
(Note: the log reduction is stated as 1 not 1.0)

Example A
                  Challenge   14 Days           28 days
Count/ml          480,000           170,000           28,000
Log (2 sig figures)     5.68        5.23        4.45
Log reduction           -           0.45        1.23

Example B
                  Challenge   14 Days           28 Days
Count/ml          480,000           170,000           28,000
Log (rounded)           6           5           4
Log reduction           -           1           2

Example A fails as a log 1 reduction is not achieved at 14 days.
Example B passes as a log 1 reduction is achieved at 14 days.

In this actual result if we apply the limit as defined in the Ph. Eur a
pass is achieved. In trying to apply greater accuracy with the use of log
values to 2 decimal places a fail is achieved. Which result would you
accept?

Brendan Lincoln
Microbiologist
Pacific Pharmaceuticals Ltd


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the scientific community.

Please take a second to visit our sponsors' web sites and say thank you for
their support of this service.

Accugenix - http://www.accugenix.com

American Type Culture Collection - http://www.atcc.org

ATS Laboratories - http://www.ats-labs.com

Biolog - http://www.biolog.com

Biomerieux - http://industry.biomerieux-usa.com

Dupont Qualicon - http://www.qualicon.com

EMD Chemicals - http://www.emdchemicals.com

EMSL Analytical, Inc. - http://www.emsl.com

Genomic Profiling Systems, Inc. - http://www.genprosys.com

MIDI, Inc. - http://www.midi-inc.com

Millipore - http://www.millipore.com

Pall - http://www.pall.com

NovaTek International - http://www.ntint.com

Raven Biological Labs - http://www.ravenlabs.com

Veltek Associates, Inc - http://www.sterile.com

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The nature of this service is to provide a medium for communication.  The
specific statements and endorsements of individuals participating in the
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the PMF, or the sponsors of the list.


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Note:
This message and any attachment are confidential and may be privileged or otherwise protected from disclosure. If you are not the intended recipient, you must not copy this message or attachment or disclose the contents to any other person. If you have received this transmission in error, please notify the sender immediately and delete the message and any attachment from your system. Pacific Pharmaceuticals Ltd does not accept liability for any omissions or errors in this message which may arise as a result of E-Mail-transmission or for damages resulting from any unauthorized changes of the content of this message and any attachment thereto. Merck does not guarantee that this message is free of viruses and does not accept liability for any damages caused by any virus transmitted therewith. 

------------------
The PMFList (http://microbiol.org/PMFList_info.htm) is operated from
The Microbiology Network (http://microbiol.org) and supported by
our sponsors (http://microbiol.org/sponsor.htm) as a service to
the scientific community.

Please take a second to visit our sponsors' web sites and say thank you for their support of this service.

Accugenix - http://www.accugenix.com

American Type Culture Collection - http://www.atcc.org

ATS Laboratories - http://www.ats-labs.com

Biolog - http://www.biolog.com

Biomerieux - http://industry.biomerieux-usa.com

Dupont Qualicon - http://www.qualicon.com

EMD Chemicals - http://www.emdchemicals.com

EMSL Analytical, Inc. - http://www.emsl.com

Genomic Profiling Systems, Inc. - http://www.genprosys.com

MIDI, Inc. - http://www.midi-inc.com

Millipore - http://www.millipore.com

Pall - http://www.pall.com

NovaTek International - http://www.ntint.com

Raven Biological Labs - http://www.ravenlabs.com

Veltek Associates, Inc - http://www.sterile.com

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The nature of this service is to provide a medium for communication.  The specific statements and endorsements of individuals participating in the discussions are not necessarily those of The Microbiology Network, Inc., the PMF, or the sponsors of the list.

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