How to Manage a Positive Allergen Residue Result when Testing Your Food Product

If you have used this commercial laboratory on numerous occasions in the past, you may have confidence that they are excellent analysts; however, you need to find out which ELISA test was conducted and the units/calibrants associated with that method if this information is not clearly provided on the analytical report. If the laboratory report does not contain information on the method used or the calibrant, you will need to contact the laboratory to get that information. The choice of the external lab can also be important. ISO-accredited labs must undergo an audit and approval process for specific methods to be included on their scope of accredited procedures. Be aware that ISO-accredited labs may not have all test methods within their ISO scope. If you want to use an ISO-accredited lab, make sure that the allergen test methods for your samples are included within their ISO scope or that that the laboratory follows a similar degree of care with procedures that are not under their ISO scope.

In addition to commercial ELISA kits, allergen analysis may also be performed by polymerase chain reaction (PCR), which detects DNA from the allergenic source, or by mass spectrometry, which detects specific proteins from the allergenic source. PCR typically has high specificity, but quantification of results and correlation to how much protein came from the allergenic source of concern can be difficult. Mass spectrometry methods hold great promise for the future, but few well-validated methods currently exist that can be used in the wide variety of ingredient and food matrices that are commonly analyzed by the food industry.

Most allergen analysis is performed using commercial ELISA kits. The level of sensitivity of those kits is typically in the low ppm range but can vary between kits. It is important to remember that the choice of calibrant affects the result (10 ppm BLG = 286 ppm NFDM) as well as the choice of protein target(s) that the kit detect. Allergen ELISA kits also have a dynamic range associated with the quantitative standards supplied with the kits. For example, the dynamic range might be 2.5–25 ppm. Many commercial laboratories will not dilute the sample if the result exceeds the upper limit of the dynamic range. Thus, you might get a result of, for example, >25 ppm. Your first question will be: How much greater than 25 ppm? Some commercial laboratories will not answer or even be able to answer this question. Some external laboratories will dilute samples, but some of these laboratories charge extra for taking that step. The FARRP Laboratory immediately dilutes samples that fall outside the dynamic range of the kit up to a maximum of 200-fold dilution (5000 ppm, or 0.5%). From a risk assessment perspective, knowing the specific, quantitative result is imperative for a thorough exposure assessment.

When you receive an unexpected result, you may want the laboratory to repeat the analysis. Some commercial laboratories will not be able to do that because they do not save any of the sample to use for a repeat analysis. Other laboratories will conduct a repeat analysis upon request, but some of them will charge again to do that test. For these reasons, we suggest that a best practice is to take duplicate or even triplicate samples, saving the extra ones at your place of business in case a repeat analysis is desired.

Could the Result Be a False Positive?

Commercial ELISA kits are typically quite specific. The manufacturers of these commercial kits assess the likelihood of cross-reactivity across a range of foods and food ingredients. Significant cross-reactivity is not often encountered except for very closely related foods such as walnut and pecan (these two nuts are botanical cousins). ELISA kit manufacturers publish written inserts that are distributed with the kits and typically include information on cross-reactivity. Commercial laboratories should be aware of cross-reactivity issues and decline to test samples that might generate false positives due to cross-reactivity in that specific test. Alternative methods should be considered if available; however, the laboratories do not always know the compositional nature of the submitted sample. Often laboratories receive unknown, food-grade powders for analysis. It is impossible for the laboratory to know what the composition of a powder may be, so it is critical to develop a good line of communication with your external laboratory. For example, we are aware of a commercial peanut ELISA kit that yielded weak positive results on samples containing high amounts of pea protein, another legume, that were reported by another commercial laboratory. This situation prompted a recall before we had the chance to inform the company that the analytical result was likely a false positive. Another best practice is to inform the external laboratory about the general composition of the test sample and ask them to verify that there are no known cross-reactivities in the ELISA kit being selected for the analysis that may affect the reliability of the results.