Microbiology: Interpretation of Results - Microchem
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Interpretation of Results

Microbiology: Interpretation of Results

Definitions:

Quantitative analysis
Analysis where a microbial count is determined

Qualitative analysis
Analysis where the presense or absence of an organism is determined

CFUs
Colony Forming Units

COA
Certificate of Analysis

  1. Why should >100g of food sample be submitted for routine testing?

This portion is a representative sample and will provide an accurate and comprehensive result.

  1. What does <10 or <100 mean?

When reporting a result where no organisms are detected, the reporting standard dictates that you cannot report zero, but that you need to report <1.

Since samples are diluted for testing purposes, this dilution also needs to be taken into account when reporting the result. For example, where no colonies are detected in a dilution of 1:10, the result would equal <10.

This result is the lowest result reportable where no organisms are detected.

Refer to the extract from ISO 7218, ‘Microbiology of Food and Animal Feeding Stuffs — General Rules for Microbiological Examinations’:

‘9.3.5.3.2 If the two dishes at the level of the test sample (liquid products) or of the initial suspension (other products) do not contain any colonies, express the result as follows:
less than 1 microorganism per millilitre (liquid products)
less than 1/d microorganisms per gram (other products)
(Where d is the dilution factor of the initial suspension)’

  1. What does >1 500 or >3 000 000 mean?

These results indicate that the microbial count reached the upper countable range of the test method. The counts are TNTC (Too Numerous to Count) and therefore are reported as more than (>) the upper countable range of the test.

Where counts exceed the recommended countable range, but are able to be counted, the result will be reported with an ‘E’ to indicate that the count is outside of the recommended countable range.

  1. Why are quantitative results reported as

The reporting format of results is dictated by the ISO Standard 7218, ‘Microbiology of Food and Animal Feeding Stuffs — General Rules for Microbiological Examinations’.

This standard dictates the following:
In liquid samples, the lowest limit of detection for absence of growth is <1.

As a dilution of solid foodstuffs is made in order to test the sample, the lowest limit of detection for absence of growth is <10.

Since a representative portion of a sample is analysed, the results indicate that no growth was obtained in the sample portion tested.
A result of ‘no growth’ or ‘zero’ is therefore not scientifically accurate because the sample dilution is not reported, or taken into account.

  1. What does the ‘E’ after a reported result mean?

The ‘E’ = Estimated
Most quantitative reference methods have a recommended range of reportable results. When counts outside of this range are reported, it is indicated using the symbol ‘E’. This count represents the actual colonies counted where the final count is outside of the recommended reporting range.

Example:
The countable range for TVC (Total Viable Count) is 30-300 CFUs/g per dilution. The results on the COA will read as follows for the following counts:
Average CFU count = 10 – Results = 10 E
Average CFU count = 350 – Results = 350 E
(For ease of interpretation, no dilution factor has been included in the example above)

Please note that we report all of our results as per international standard, ISO 7218, ‘Microbiology of Food and Animal Feeding Stuffs – General Rules for Microbiological Examinations’.

Please refer to point 9.3.5.3.1 and 9.3.5.3.2, wherein the following is stated with regards to calculation and reporting of results (see below):

‘9.3.5 Expression of results
9.3.5.3 Estimated counts
9.3.5.3.1 If the two dishes, at the level of the test sample (liquid products) or of the initial suspension (other products), contain less than 15 colonies, calculate the arithmetical mean y of the colonies counted on two dishes.
Express the result as follows:
for liquid products: estimated number of microorganisms per millilitre NE = y
for the other products: estimated number of microorganisms per gram NE = y/d
(where d is the dilution factor of the initial suspension).’

9.3.5.3.2 If the two dishes at the level of the test sample (liquid products) or of the initial suspension (other products) do not contain any colonies, express the result as follows:
less than 1 microorganism per millilitre (liquid products)
less than 1/d microorganisms per gram (other products)
(where d is the dilution factor of the initial suspension).’

  1. What are preliminary results, and why are they not signed?

Preliminary results indicate analyses which are complete. They may also show results which have not been completed, and are pending.

Preliminary results can be provided upon request of the client. They may be used by the client to establish further testing requirements. They may be used by the laboratory to convey results to the client before the final report can be issued.

When preliminary results are provided in the form of a Certificate of Analysis, they will not be signed. A signature indicates that the results and request have been verified by a technical signatory, and that the results are complete.

  1. Why are presumptive and confirmed results reported separately?

Certain reference methods stipulate that samples with presumptive positive results require further confirmation. Confirmation is therefore a obligation to complete the method, and report the final result as ‘Positive’ or ‘Detected’.

Results which have no presumptive colonies are complete according to the method. No further confirmation can be performed, and no additional result or charge for confirmation is therefore needed for these samples.

The results, and the additional cost associated with performing the confirmations are therefore separated.

  1. What does NR mean?

NR indicates a non-reportable result, and will be indicated in the results column of the COA. Results may not be reportable for a number of reasons.

NR is reported when results obtained are not microbiologically sound, for example, the counts are not within laboratory acceptable limits; or the food matrix affected the test media, resulting in colonies enumerated not portraying typical bacterial characteristics, or results.

In the case of NR, the cost for that particular analysis is removed and it is recommended to re-submit that particular sample for analysis so as to obtain conclusive results.