2.9

Skip to 0 minutes and 1 secondThere are many aspects of hazard control that ensure our food / feed chain is safe. However, we will focus on the role detection methods play by looking at two techniques used to identify biological hazards such as salmonella and marine bio toxins. Both hazards are invisible to the naked eye. You can't usually smell them or taste them either. Therefore, their presence or absence in a food must be demonstrated by food testing to ensure food safety. We're going to consider the characteristics that the detection test must possess to be appropriate for use. Speed of acquisition of results, in other words, a rapid test and ease of use are fundamental features of many new food safety tests.

Skip to 0 minutes and 43 secondsHowever, prior to use it is critical that new tests are comparatively evaluated or validated against existing methods. This will ensure their effectiveness in the intended test setting. The following key characteristics of a detection method would be assessed by in-house experiments, or by interlaboratory trials, detection sensitivity, detection specificity, accuracy, repeatability, and reproducibility. We are now going to consider each of these test characteristics in more detail. And we will learn how the first two test characteristics would be determined in practise. I'm going to take you through a worked example to calculate detection sensitivity and specificity of a novel antibody-based lateral flow assay to detect toxin in shellfish.

Skip to 1 minute and 36 secondsAnd then you will get the opportunity to calculate these characteristics for an IMS-PCR method to detect salmonella in pork from experimental results. The first two test characteristics that would be assessed deal with how well the test is able to distinguish contaminated food from non-contaminated food. And these are determined by research studies. Detection sensitivity is the probability that a test will give a positive result, if the contaminant is present. The more sensitive a test, the fewer false negative results it produces. Detection specificity is the probability that a test will give a negative result, if the contaminant is not present. The more specific a test, the fewer false positive results it produces.

Skip to 2 minutes and 26 secondsThe next three test characteristics reflect how well the test method performs day to day in a laboratory. Accuracy, a test method is said to be accurate when it measures what it is supposed to measure. In other words, the test value approaches the true value of the substance being measured. Repeatability, or precision, a test is said to be precise or reliably reproducible when repeated analyses on the same sample under the same conditions over a short period of time gives similar results, and the amount of variation is small. Reproducibility, refers to variation in measurements made on a subject under changing conditions. For example, different measurement instruments being used, different operators over a longer period of time, or even in different laboratories.

Skip to 3 minutes and 22 secondsAccuracy and precision would be periodically monitored by laboratory personnel. Ideally, a test should have high detection sensitivity. In other words, be able to detect low levels of a target analite. And high detection specificity, in other words, not give a positive if the target analite is not present. Sometimes there have to be trade offs in terms of sensitivity and specificity. As it is not always possible to achieve both. False negative and false positive results may consequently occur. The four possible test result categories are illustrated in the following table. A true positive is obtained if a contaminated food sample yields a positive test result.

Skip to 4 minutes and 12 secondsA false positive result is obtained if a food sample not contaminated by the hazard yields a positive test result. A false negative result is obtained if a contaminated food sample yields a negative test result. A true negative result is obtained if a food sample not contaminated by the hazard yields a negative result. Spiking experiments are typically carried out to determine detection specificity and sensitivity. In a spiking experiment, an amount, or a varying amount of particular food hazard of interest will be deliberately added to some food samples and not added to others. Testing will proceed, and the results obtained will be entered into a two by two table to enable calculation of sensitivity and specificity using these equations.

Skip to 5 minutes and 5 secondsHere is a worked example demonstrating how a novel lateral flow test for shellfish toxin performed when muscle samples were tested. And how sensitivity and specificity are calculated. 80 muscle samples were tested by a recently developed antibody-based lateral flow assay, or LFA. 40 of the muscles were spiked with a particular shellfish toxin and 40 had nothing added. Samples were presented to the person carrying out the test simply as number coded samples without any further details, in other words, they were tested blind. Results obtained are shown in the following table. Let me talk you through the spiking experiment results. The lateral flow assay detected toxin, i.e. gave a positive result in 35 of the 40 toxin spiked muscle samples.

Skip to 6 minutes and 0 secondsThese are the true positives. But it failed to detect toxin in the remaining five toxin spiked muscle samples. These are therefore, false negatives. However, the lateral flow assay also gave a positive test result for 10 muscle samples which hadn't had a toxin added. These are, therefore, false positives, and only gave a negative test result for 30. These are true negatives rather than 40 of the non-spiked samples. Sensitivity and specificity can then be calculated using the equations. In this example, the existence of both false positive and false negative results has adversely affected the detection sensitivity and detection specificity of the lateral flow assay respectively, as demonstrated by the calculated values. A perfect test would have 100% sensitivity and specificity, i.e.

Skip to 6 minutes and 58 secondsno false negative or positive results.

Key characteristics of detection methods

The availability of a new rapid method alone does not mean that it is immediately going to be adopted as a standard method for wide scale testing.

Irrespective of the scientific basis of any detection method, the test needs to be properly validated before being introduced onto the market.

The key characteristics of a food safety detection method are:

  • Detection sensitivity
  • Detection specificity
  • Accuracy
  • Ease of use
  • Speed
  • Reproducibility/repeatability

In the video, a lateral flow test for detecting toxins in shellfish is considered in an example of how to determine detection sensitivity and specificity.

You can find out more about detection methods by reading two articles by Matabaro et al (2017) and Woan-Fei Law et al (2015). These are also found below in the downloads and ‘see also’ section.

  • Are you aware of other recent articles that you can share?
  • Have you used any of these methods before?

Please discuss your findings or thoughts with other learners in the comments area.

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This video is from the free online course:

Tackling Global Food Safety

Queen's University Belfast