30 PRESERVATIVES
Step 6: Calculate log reduction or % kill
Some methods require a log reduction, almost always determined as a common logarithm or log base 10 (log10
) such as
Pharmacopeia Preservative Efficacy Tests, while others require % kill to be reported, as is usually the case for antimicrobial cleaners and disinfectants. The two methods use the same data but with different calculation methods. The log10 reduction compares the number of organisms surviving against the original number added while the % kill compares the number of organisms killed against the original number added. The calculation methods are shown in Table 2. There is a direct correlation between reduction and % kill as shown by the
log10 data in Table 3.
Step 7: Report the results Test methods generally require a number of factors be met to ensure the test is valid. These will include such things as the correct number of organisms in the challenge, the validity of the neutraliser and the survival and recovery of the test organisms from untreated control samples. If these requirements are met a report
can be prepared showing the calculated results. If the method contains pass criteria these can be compared to the results achieved and a statement of compliance with the test method can be made, such as “The sample tested meets Criteria A of the BP Efficacy of Antimicrobial Preservation”. If, however, the test method contains no pass criteria the decision of what constitutes a pass is to be made between interested parties, making a compliance statement difficult. Laboratories accredited to ISO 17025 [ISO 17025 - 15 General Requirements for the competence of testing and calibration laboratories12
] are required to calculate
uncertainty of measurements for all analysis conducted. This can be calculated but is not generally taken into account when reporting antimicrobial efficiency tests unless specifically requested. However, the 2017 version of the standard requires that measurement of uncertainty be taken into account when making compliance statements and only permits them if: l The measurement results fall within the specification limits by an amount at least equivalent to the uncertainty of measurement; or
l The measurement results fall within the specification limits and the uncertainty of measurement is within the maximum permissible uncertainty prescribed in the specification; or
l The test specification defines the compliance decision rule to be used and the measurement results meet the
PERSONAL CARE EUROPE
ISO 11930 Criteria A
ISO 11930 Criteria B
Table 4: Acceptance criteria Test Method Log reduction of surviving organisms Bacteria
USP BP/EP Criteria A BP/EP Criteria B
CTFA (PCPC)
CTPA
≥2 by 14 days and no increase thereafter
≥2 by 2 days and ≥3 by 7 days No increase thereafter
≥3 by 14 days No increase thereafter
>3 by 7 days and no increase thereafter
≥3 by 2 days and no increase thereafter
≥3 by 7 days No increase thereafter
≥3 by 14 days No increase thereafter
Moulds & Yeast
No increase in count throughout test
≥2 after 14 days and No increase thereafter
≥1 after 14 days and No increase thereafter
≥1 after 7 days and No increase thereafter
≥2 after 14 days and No increase thereafter
Yeast: ≥1 after 7 days and no increase thereafter. Mould: no increase at day 14 and ≥1 after 28 days
Yeast: ≥1 after 14 days and no increase thereafter. Mould: no increase in count throughout the test
Table 5: Reduction criteria (log10) for bacteria for different test methods. Test Method BP Criteria A
2 days 7 days
BP Criteria B USP
ISO 11930 Criteria A ISO 11930 Criteria B
specified criteria; or
l The customer and facility have agreed to a compliance decision rule.
Laboratories may not make a statement
of conformity based on an agreement with the customer If the report is for the purpose of regulatory compliance. So, if the test report is part of the development or quality assurance process, a compliance statement may be made with no reference to measurement of uncertainty. But, if the report will be used to meet regulatory compliance, then either the uncertainty for each measurement must be calculated and used to determine if the results meet the requirements set out above, or no compliance statement may be included on the report.
What do the test reports mean? The best way is to look at a number of test procedures and see what the results actually mean. Preservative Efficacy Tests (PET) may be conducted to a number of standards. They require that a range of defined micro- organisms are added to the sample under
14 days 21 days
28 days
23 33 3 –– 33 3 –– 22 2 –3 33 3 –– 33 3
test and the sample is assayed at required times for surviving organisms. There are defined outcomes set for a reduction in the number of surviving organisms with time. The tests are designed to be reproducible and comparable and they gauge the effectiveness of the preservative system to control representative species. Their end points do not specify product sterility of the challenged product and they do not simulate in-use conditions or effects of packaging and as such only form part of the safety testing of a product. Pass criteria require minimum reductions in the number of surviving organisms calculated as log reductions. The required reductions for various Standard Test Methods are shown in Table 4.
Standard PET test tests require inoculating separate samples of the product under test once with individual organisms and measuring the number of surviving organisms at defined times. This method was developed for assessing preservatives in pharmaceuticals rather than cosmetics but differences in pack sizes, period after opening and use patterns lead
April 2020
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