Limitations of point-of-care testing for low SARS CoV-2 loads: Insights for future pandemics.

Background:  The emergence of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has seen a surge in the development of diagnostic assays. However, the performance of antigen point-of-care tests (Ag-POCTs) on samples with low viral load has not been evaluated.

Aim:  To evaluate the accuracy of three World Health Organization (WHO) certified Ag-POCTs in comparison to the reverse transcription polymerase chain reaction (RT-PCR) technique.

Setting:  The study was conducted at Pietersburg Hospital Limpopo, South Africa between March 2020 and April 2023.

Methods:  A total of 371 SARS-CoV-2 nasopharyngeal samples from the National Health Laboratory Service were tested using Ag-POCTs from Abbott Panbio, Roche RDT and SD Biosensor, following manufacturer instructions. All samples had RT-PCR results with Ct values between 13 and 45. Reverse transcription polymerase chain reaction results were compared and correlated with Ag-POCT results.

Results:  Of the 371 samples, the SD Biosensor Standard Q test kit detected the most positive isolates 166 (44.7%), followed by the Abbott Panbio. A total of 153 (41.2%) positives, while the Roche SD detected 134 (36.1%) samples. High viral load (Ct 25) sensitivity and specificity exceeded 77%, while intermediate (Ct 25-35) and low viral load (Ct 35) sensitivity and sensitivity dropped to 32% and 7%, respectively.

Conclusion:  The performance rapid antigen tests was low on samples with low viral load with results markedly different from the manufacturer's reported performance.Contribution: Rapid antigen tests should not be used alone for diagnosis, especially in samples with low viral load.