Point-of-care serological assays for delayed SARS-CoV-2 case identification among health-care workers in the UK: a prospective multicentre cohort study

Abstract

Background: Health-care workers constitute a high-risk population for acquisition of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection. Capacity for acute diagnosis via PCR testing was limited for individuals with mild to moderate SARS-CoV-2 infection in the early phase of the COVID-19 pandemic and a substantial proportion of health-care workers with suspected infection were not tested. We aimed to investigate the performance of point-of-care and laboratory serology assays and their utility in late case identification, and to estimate SARS-CoV-2 seroprevalence.

Methods: We did a prospective multicentre cohort study between April 8 and June 12, 2020, in two phases. Symptomatic health-care workers with mild to moderate symptoms were eligible to participate 14 days after onset of COVID-19 symptoms, as per the Public Health England (PHE) case definition. Health-care workers were recruited to the asymptomatic cohort if they had not developed PHE-defined COVID-19 symptoms since Dec 1, 2019. In phase 1, two point-of-care lateral flow serological assays, the Onsite CTK Biotech COVID-19 split IgG/IgM Rapid Test (CTK Bitotech, Poway, CA, USA) and the Encode SARS-CoV-2 split IgM/IgG One Step Rapid Test Device (Zhuhai Encode Medical Engineering, Zhuhai, China), were evaluated for performance against a laboratory immunoassay (EDI Novel Coronavirus COVID-19 IgG ELISA kit [Epitope Diagnostics, San Diego, CA, USA]) in 300 samples from health-care workers and 100 pre-COVID-19 negative control samples. In phase 2 (n=6440), serosurveillance was done among 1299 (93·4%) of 1391 health-care workers reporting symptoms, and in a subset of asymptomatic health-care workers (405 [8·0%] of 5049).

Findings: There was variation in test performance between the lateral flow serological assays; however, the Encode assay displayed reasonable IgG sensitivity (127 of 136; 93·4% [95% CI 87·8-96·9]) and specificity (99 of 100; 99·0% [94·6-100·0]) among PCR-proven cases and good agreement (282 of 300; 94·0% [91·3-96·7]) with the laboratory immunoassay. By contrast, the Onsite assay had reduced sensitivity (120 of 136; 88·2% [95% CI 81·6-93·1]) and specificity (94 of 100; 94·0% [87·4-97·8]) and agreement (254 of 300; 84·7% [80·6-88·7]). Five (7%) of 70 PCR-positive cases were negative across all assays. Late changes in lateral flow serological assay bands were recorded in 74 (9·3%) of 800 cassettes (35 [8·8%] of 400 Encode assays; 39 [9·8%] of 400 Onsite assays), but only seven (all Onsite assays) of these changes were concordant with the laboratory immunoassay. In phase 2, seroprevalence among the workforce was estimated to be 10·6% (95% CI 7·6-13·6) in asymptomatic health-care workers and 44·7% (42·0-47·4) in symptomatic health-care workers. Seroprevalence across the entire workforce was estimated at 18·0% (95% CI 17·0-18·9).

Interpretation: Although a good positive predictive value was observed with both lateral flow serological assays and ELISA, this agreement only occurred if the pre-test probability was modified by a strict clinical case definition. Late development of lateral flow serological assay bands would preclude postal strategies and potentially home testing. Identification of false-negative results among health-care workers across all assays suggest caution in interpretation of IgG results at this stage; for now, testing is perhaps best delivered in a clinical setting, supported by government advice about physical distancing.

Funding: None.

Figure 1

Results of health-care workers and negative controls tested with IgG ELISA and Encode and Onsite split IgM/IgG antibody lateral flow serological assays in London, UK, from April to June, 2020 Phase 1: comparison of matched samples tested with the Onsite CTK Biotech COVID-19 split IgG/IgM Rapid Test (CTK Biotech, Poway, CA, USA) and Encode SARS-CoV-2 split IgM/IgG One Step Rapid Test Device (Zhuhai Encode Medical Engineering, Zhuhai, China) LFAs and ELISA (EDI Novel Coronavirus COVID-19 IgG ELISA kit [Epitope Diagnostics, San Diego, CA, USA]). 100 historical serum samples were evaluated to assess specificity (Table 2, Table 3). Equivocal results were retested with the Abbott SARS-CoV-2 IgG (anti-nucleocapsid) chemiluminescent microparticle immunoassay (Abbott Laboratories, Lake Bluff, IL, USA). Phase 2: further analysis of LFA testing offered to all symptomatic health-care workers, including initial LFAs tested in phase 1 (n=300) and phase 2 (n=1299). Further analysis of asymptomatic health-care workers with LFA testing (n=405). LFA=lateral flow serological assay. PHE=Public Health England.

Figure 2

Distribution of visual scores for Encode (A) and Onsite (B) LFAs against optical density readings of ELISA-matched samples Readings of the Onsite CTK Biotech COVID-19 split IgG/IgM Rapid Test (CTK Biotech, Poway, CA, USA) and the Encode SARS-CoV-2 split IgM/IgG One Step Rapid Test Device (Zhuhai Encode Medical Engineering, Zhuhai, China) LFAs at 15 min plotted against ELISA (EDI Novel Coronavirus COVID-19 IgG ELISA kit [Epitope Diagnostics, San Diego, CA, USA]) optical density for all health-care worker matched samples (n=300). Scoring correlates to a negative (0), very weak positive (1), weak positive (2), medium positive (3), and strong positive (4) reading. Reference photographs selected from mean optical density value cassette for each score. SDs of Encode lateral flow serological assay values: negative (0·09), very weak (0·21), weak (0·26), medium (0·21), and strong (0·20) visual scores. Dunn's pairwise tests showed no significant difference between visual scores, comparing very weak to medium (p=0·35), medium to strong (p=0·064), very weak to weak (p=1·00), and weak to medium (p=1·00) visual scores. All other pairwise comparisons had a significant relationship with optical density (p<0·0001). SDs of Onsite lateral flow serological assay values: negative (0·17), very weak (0·21), weak (0·23), medium (0·29), and strong (0·30) visual scores. Dunn's pairwise tests showed no significant difference between visual scores comparing very weak to weak (p=1·00), weak to medium (p=0·062), and medium to strong (p=1·00) visual scores. All other pairwise comparisons had a significant relationship with optical density (p<0·0043).