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We intuitively understand that if we test the same product twice, the microbial counts are not going to be exactly identical. However, what might surprise you is just how much repeated microbial test results from the same product can vary. The answer for these different results lies in a concept known as measurement uncertainty.
The measurement uncertainty refers to the idea that rather than a test result being the true value, there is a range of values in which the true value lies. That test result is not likely to match the true value, but it is likely to fall within this range. But what is the range? There are a number of ways to calculate and express it, but a common value used is two standard deviations on either side of the test result. The purpose of this uncertainty value is to capture the doubt about a measured value, e.g. a microbiological tests count.
With that in mind, let's look at an example. Many standards for microbial testing include reproducibility values, which describe the expected range for tests results coming from different laboratories. In our example a food manufacturer wants a Standard Plate Count (SPC) for their ready-to-eat product. Being very cautious, they get the same test done twice at two different laboratories, both of which happen to carry out the SPC test using the method ISO 4833-1. The results come back: 100,000 CFU/g at the first laboratory but only 50,000 CFU/g at the second. The second result is half the first! However, are they really all that different? The reproducibility of the method ISO 4833-1 states that the difference between two tests results should not be greater than 0.45 in log(N). Let's do the calculations:
The log(N) value of the first laboratory result: log(100,000) = 10^5
The upper limit: 10^(5+0.45) = 280,000
The lower limit: 10^(5-0.45) = 36,000
As long as the second result is between 36,000 and 280,000 CFU/g then the difference is acceptable as per ISO 4833-1. We can see that the second result of 50,000 falls well within this range.
Although surprising, such variation is not uncommon with microbiological tests.
References:
GUM: Guide to the Expression of Uncertainty in Measurement
ISO 4833-1:2013 Microbiology of the food chain — Horizontal method for the enumeration of microorganisms Part 1: Colony count at 30 °C by the pour plate technique