New Study Identifies Impact of Sample Processing on Sensitivity of Rapid Antigen Detection Tests for SARS-CoV-2

A new study aimed at identifying the impact of sample processing on the sensitivity of rapid antigen detection (RAD) tests for SARS-CoV-2 has shown that selecting the appropriate inactivation methods and viral transport media (VTM) solutions is necessary during reagent development, performance evaluation, and clinical application.

The study published by the Chinese Medical Association (Beijing, China) explored the effect of different inactivation methods, VTM solutions, and sample preservation on the sensitivity of four RAD kits based on two SARS-CoV-2 strains. According to the results, heat inactivation may not be a suitable inactivation method for SARS-CoV-2 testing, especially for clinical testing and performance evaluation. This finding was consistent with a previous study reporting that heat inactivation harmed the efficiency of RT-PCR assay for SARS-CoV-2. Heat inactivation could be one of the possible factors of false-negative results in the RT-PCR assay of SARS-CoV-2 detection. Heat inactivation destroys the structure of the RNA and protein of SARS-CoV-2. RT-PCR assay can detect the nucleotide of SARS-CoV-2, while the RAD test can detect the antigen of SARS-CoV-2. Thus, heat inactivation could substantially affect the sensitivity of RAD kits.

Chemical inactivators might contribute a lot in protecting the laboratory personnel in charge of detecting SARS-CoV-2, especially nucleic acid detection because chemical inactivation can inactivate clinical samples containing SARS-CoV-2. Similarly, there might be some chemicals existing in VTMs that can denature and inactivate protein, thus affecting the performance of RAD kits. The VTM solution's impact on the RAD kits might be mainly conferred by the detection antibodies used in the kits, which utilize different epitopes to recognize the antigens in other kits. During sample inactivation, the VTM solution might change the tertiary structure of the antigen in the sample and cause it to degrade. Thus, the recognition between antigen and detection antibodies in the following detection process might greatly vary. Therefore, the reaction of different reagents to the same VTM was different, indicating that choosing the appropriate VTM solutions for RAD testing is necessary.

Additionally, the results showed that, along with the extension of preservation time and the increase of freeze–thaw cycles, the detection value of RAD kits slightly decreased, but most of them were still in the range of detection. A previous study had shown that the copy number of the DNA target decreased after 10 freeze-thaw cycles, which affected the performance of the droplet digital PCR. Also, there was no access to HIV viral load testing when the whole blood was stored in EDTA tubes or plasma preparation tubes for more than 6 h at 25 °C. Therefore, reasonable storage temperature and preservation time of samples are essential measures to ensure the accuracy of test results.

Thus, the study revealed that different virus inactivation methods and VTM solutions significantly impact the sensitivity of RAD kits. Therefore, it is necessary to select the appropriate inactivation methods and VTM solutions. Furthermore, as most RAD kits remain stable under different storage conditions, it is not required to consider the samples' preservation conditions. According to the results, during reagent development, performance evaluation, and clinical application, samples should not be treated by heat inactivation or unqualified VTM solutions before evaluating the performance of clinical samples, such as the lowest detection limitation. Moreover, samples should not be treated by heat inactivation or unqualified VTM solutions in clinical trials. In addition, attention should also be given to the warnings of the approved reagents, such as sample collection and preservation. To sum up, selecting VTM solutions might be more convenient, as they can effectively crack and inactivate viruses without affecting antigen testing.

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