Understanding the Variations in Antigen and PCR Test Results: Why Rapid Antigen Tests Can Show Negative Results While PCR is Positive

Rapid antigen tests and Reverse Transcription Polymerase Chain Reaction (RT-PCR) are both widely used to diagnose SARS-CoV-2 infections. However, there can be discrepancies in their results, leading to confusion. This article aims to clarify why sometimes a rapid antigen test shows a negative result while an RT-PCR test is positive. We will explore the underlying reasons, including the time frame for antigen buildup, potential for false negatives and positives, and specific testing methods.

Key Takeaways

The rapid antigen test may show a negative result before enough viral antigens accumulate. Antibody tests are not diagnostic but can be used for screening. PCR tests are highly sensitive but require specialized equipment and trained personnel. Rapid antigen tests are simpler and more accessible. Viral load affects the accuracy of antigen detection.

Background

During the ongoing global pandemic, ensuring the accurate diagnosis of SARS-CoV-2 is crucial to limiting its spread and providing proper clinical management for those infected with COVID-19. Real-time reverse transcription polymerase chain reaction (RT-qPCR) remains the gold standard for diagnosing acute SARS-CoV-2 infections. However, it has limitations such as the need for specialized equipment and trained personnel, along with its higher cost. As a result, rapid antigen detection tests have been developed and promoted as an alternative, especially for large-scale screening in resource-limited settings.

Objectives

The main objective of this study was to evaluate the performance of the Coris COVID-19 Ag Respi-Strip test, a rapid immunochromatographic test used for detecting SARS-CoV-2 antigen, compared to the RT-qPCR method.

Methods

148 nasopharyngeal swabs were tested. The results from the Coris COVID-19 Ag Respi-Strip test were compared to those from RT-qPCR.

Results

Among the 106 positive RT-qPCR samples, 32 were detected by the rapid antigen test, giving an overall sensitivity of 30.2%. Importantly, all samples that tested positive with the rapid antigen test were also positive when tested with the RT-qPCR.

Discussion

The study highlights that higher viral loads are associated with better antigen detection rates. However, the overall poor sensitivity of the COVID-19 Ag Respi-Strip test suggests that it cannot be used alone for frontline diagnosis of COVID-19 infections.

Understanding the Delay in Antigen Buildup

The reasons why a rapid antigen test can show negative results while an RT-PCR test is positive are multifaceted. One key factor is the time frame for antigen buildup after viral exposure. It can take 7-10 days for enough viral antigens to accumulate in the body to be detected by the rapid antigen test.

False Negatives and Positives

False negatives and false positives are inherent in any medical test, including both antigen and PCR tests. False negatives occur when an antigen test incorrectly indicates that a person is not infected, while false positives indicate that an antigen test incorrectly suggests infection in a non-infected individual.

PCR vs. Rapid Antigen Tests: Complementary Uses

PCR tests are highly sensitive and can detect even low levels of viral RNA, making them the gold standard for confirming SARS-CoV-2 infection. However, they are complex, require trained personnel, and are expensive. In contrast, rapid antigen tests are simpler, more accessible, and can be performed quickly in various settings.

Rapid antigen tests are particularly useful as screening tools for large-scale testing. For instance, those who are negative by rapid antigen tests can be further tested with RT-qPCR. Conversely, if a rapid antigen test is positive, it can be sufficient to diagnose COVID-19, thereby reducing the burden on PCR testing facilities.

Conclusion

The performance of rapid antigen tests for diagnosing SARS-CoV-2 infections is influenced by factors such as viral load and the time frame for antigen accumulation. While these tests have practical advantages, their lower sensitivity sometimes results in false negatives, which can be confirmed by RT-qPCR. Understanding these nuances is crucial for optimizing large-scale testing strategies and enhancing the overall accuracy of SARS-CoV-2 diagnosis.