POINT-OF-CARE ANTIBODY TESTS for COVID-19: FIELD-BASED PERFORMANCE, SOUTH AFRICA

Background: SARS-CoV2 antibody testing is an important auxillary test especially for retrospective diagnosis or in patients with long COVID-19 or multisystem inflammatory syndrome of childhood. Epidemiological serology studies may also assist public health planning. Access to formal laboratory testing is not universal in many low-and middle-income (LMIC) countries and rapid lateral flow antibody tests are an attractive alternative. Performance of these tests has been inconsistent. A large-scale study was undertaken in South Africa, during the beta and delta waves, to assess the field-based performance of rapid point of care (POC) COVID-19 antibody tests. Methods: Symptomatic, ambulatory persons under investigation (PUIs) aged 18 years and older, presenting for SARS-CoV-2 diagnosis at public health facilities in three provinces, South Africa were enrolled at baseline. All patients completed a questionnaire regarding symptoms. Nasopharyngeal swabs were taken and processed for SARS-CoV-2 PCR testing using a GeneXpert (Cepheid, USA), or manual assay (ThermoFisher TaqPath assay or Seegene Allplex assay) on a real-time platform at routine accredited National Health Laboratory Service laboratories as per routine national protocols. Concomitantly, trained study staff performed three facility-based POC lateral flow antibody tests on a on a fingerstick sample and blood was collected for formal serology. POC tests were selected following a rapid in-laboratory evaluation. Asymptomatic contacts of people with confirmed COVID-19 were recruited into the asymptomatic study arm and rapid tests and PCR were performed. PCR and rapid positive patients and 500 negative controls were followed up at 5-14 days. Antibody tests were compared with formal serology performed on 2 platforms-Euroimmun (Euroimmun, Lubeck) IgA and IgG anti-S antibodies and Abbott Architect IgG test. Results: The sensitivity (S), specificity (Sp), positive (PPV) and negative predictive (NPV) values of tests for PUIs and contacts were calculated (Table 1)∗. Analyses using serology as a reference are forthcoming. Conclusion: Compared with PCR, performance of rapid POC COVID-19 antibody tests was poor with low sensitivity. This may reflect the patient cohort tested as humoral responses typically develop from day 7-14. The tests are unlikely to be useful for acute diagnosis but sensitivity may improve at later timepoints and further follow up data will be analysed by duration of symptom onset, severity of symptoms and wave (beta versus delta).

UTILITY of COVID-19 POINT-OF-CARE ANTIGEN TESTS in LOW-MIDDLE INCOME SETTINGS

Background: Access to SARS-CoV-2 polymerase chain reaction (PCR) testing is a bottleneck globally, especially in low-and middle-income countries (LMICs). Reliable point-of-care (POC) diagnostics for coronavirus disease 2019 (COVID-19) are cheaper and easier to scale-up than PCR especially in LMICs, and will facilitate interruption of transmission. We report the field-based effectiveness of rapid point-of-care (POC) antigen COVID-19 tests during the beta and delta waves, in South Africa. Methods: We enrolled symptomatic, ambulatory persons under investigation (PUIs) aged 18 years and older, presenting for SARS-CoV-2 diagnosis at public health facilities in three provinces, South Africa. All patients completed a questionnaire regarding symptoms. Nasopharyngeal swabs were taken and processed for SARS-CoV-2 PCR testing using either GeneXpert (Cepheid, USA), or with a manual assay (ThermoFisher TaqPath assay or Seegene Allplex assay) on a real-time PCR platform at routine, accredited National Health Laboratory Service laboratories, as per routine national protocols. Concomitantly, trained study staff performed three facility-based POC antigen tests on a nasal/nasopharyngeal swab, as recommended by the manufacturer. Asymptomatic contacts of people with confirmed COVID-19 were recruited into the asymptomatic study arm and rapid tests and PCR were performed. The sensitivity (S), specificity (Sp), positive (PPV) and negative predictive (NPV) values of tests for PUIs and contacts were calculated using PCR as the reference standard. Results: Between Oct 2020-2021 1816 participants were enrolled;472 (26%) tested PCR or rapid test positive;235 positives (49.8%) and 532 negatives were followed up at 5-14 days;574 asymptomatic contacts were enrolled, of which 21 (3.7%) were PCR positive. Performance of the three antigen tests are shown in Table 1∗. Conclusion: In a real world setting, during the beta and delta waves, compared with PCR the sensitivity of rapid antigen tests ranged from 35-68%. This may reflect low viral loads at diagnosis. Further work will compare antigen test performance in patients with high versus lower cycle threshold (Ct) values. Meanwhile, PCR testing capacity needs urgent scale-up in LMICs and improved POC diagnostics are needed to facilitate COVID-19 diagnosis in LMICs.

Diagnosis of COVID-19: Considerations, controversies and challenges.

Coronavirus disease 2019 (COVID-19) due to a novel virus, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is a global pandemic that has resulted in over 1.5 million confirmed cases and close to 100 000 deaths. In the majority of symptomatic cases, COVID-19 results in a mild disease predominantly characterised by upper respiratory tract symptoms. Reverse transcription polymerase chain reaction (RT-PCR) using a nasopharyngeal sample is the mainstay of diagnosis, but there is an ~30% false negative rate early in the disease and in patients with mild disease, and therefore repeat testing may be required. RT-PCR positivity can persist for several days after resolution of symptoms. IgM and IgG antibody responses become positive several days after the onset of symptoms, and robust antibody responses are detectable in the second week of illness. Antibody-based immunoassays have a limited role in the diagnosis of early symptomatic disease. However, their incremental benefit over RT-PCR in the first 2 weeks of illness is currently being clarified in ongoing studies. Such assays may be useful for surveillance purposes. However, their role in potentially selecting individuals who may benefit from vaccination, or as a biomarker identifying persons who could be redeployed into essential employment roles, is being investigated. Rapid antibody-based immunoassays that detect viral antigen in nasopharyngeal samples are being developed and evaluated.