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The Pharmaceutical Microbiology Forum Email List <[log in to unmask]>
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Mon, 8 May 2006 14:15:08 +1200
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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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