Assessment of SARS-CoV-2 serological tests for the diagnosis of COVID-19 through the evaluation of three immunoassays: Two automated immunoassays (Euroimmun and Abbott) and one rapid lateral flow immunoassay (NG Biotech)

Abstract

Background: The emergence of new SARS-CoV-2 has promoted the development of new serological tests that could be complementary to RT-PCR. Nevertheless, the assessment of clinical performances of available tests is urgently required as their use has just been initiated for diagnose.

Objectives: The aim of this study was to assess the performance of three immunoassays for the detection of SARS-CoV-2 antibodies.

Methods: Two automated immunoassays (Abbott SARS-CoV-2 CLIA IgG and Euroimmun Anti-SARS-CoV-2 ELISA IgG/IgA assays) and one lateral flow immunoassay (LFIA NG-Test® IgG-IgM COVID-19) were tested. 293 specimens were analyzed from patients with a positive RT-PCR response, from patients with symptoms consistent with COVID-19 but exhibiting a negative response to the RT-PCR detection test, and from control group specimens. Days since symptoms onset were collected from clinical information sheet associated with respiratory tract samples.

Results: Overall sensitivity for IgG was equivalent (around 80 %) for CLIA, ELISA and LFIA. Sensitivity for IgG detection, >14 days after onset of symptoms, was 100.0 % for all assays. Overall specificity for IgG was greater for CLIA and LFIA (more than 98 %) compared to ELISA (95.8 %). Specificity was significantly different between IgA ELISA (78.9 %) and IgM LFIA (95.8 %) (p < 0.05). The best agreement was observed between CLIA and LFIA assays (97 %; k = 0.936).

Conclusion: Excellent sensitivity for IgG detection was obtained >14 days after onset of symptoms for all immunoassays. Specificity was also excellent for IgG CLIA and IgG LFIA. Our study shows that NG-Test® is reliable and accurate for routine use in clinical laboratories.

Keywords: Automated immunoassays; COVID-19; Lateral flow immunoassay; Performance; SARS-CoV-2.

Fig. 1

Seropositivity of tested specimens with ELISA Euroimmun and CLIA Abbott assays. Seropositivity analysis in 95 presumed negatives control samples (cross-reactivity samples, march 2019 samples, pregnant women samples, patients with RF samples), 57 samples from 52 patients with RT-PCR negative relative to days from symptom onset (≤7 days; 8-14 days; >14 days) and 141 samples from 82 patients with RT-PCR positive relative to days from symptom onset. Blue circles correspond to sera from patients exhibiting a positive RT-PCR result. Green circles correspond to sera from patients with negative RT-PCR result. Ochre circles correspond to sera from individuals for whom RT-PCR detection has not been performed. The black line represents the median of ratio. (+): number of seropositive sera; (n): total number of specimens tested. A) Seropositivity with ELISA Euroimmun assay. Dashed grey line represents cutoff for positivity (ratio ≥1.1). Dotted purple line corresponds to cutoff for negativity (ratio <0.8). B) Seropositivity with CLIA Abbott assay. Dashed grey line represents cutoff for positivity (ratio ≥1.4). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).

Fig. 2

Anti-SARS-CoV-2 antibodies seroconversion profiles for seven individuals. X-axis: time from symptoms onset. Y-axis: interpretation ratio of two semi-quantitative immunoassays. Dotted black line represents the day of positivity of LFIA NG-Test® IgG-IgM COVID-19. The cutoff for positivity with ELISA Euroimmun assay is ≥1.1 (dotted blue line) and the cutoff for positivity with CLIA Abbott assay is ≥1.4 (dotted red line). Patients 1, 2, 4 and 6 developed a prolonged immune response one month after symptoms onset and up 64 days for patient 4. Patients 1, 2 and 7 had early seroconversion in the second week after symptoms onset and patient 5 had already seroconversion in the first week. Patient 3 had a seroconversion in the third week after symptoms onset. Patient 5 produced fewer antibodies compared to other patients and notably IgA production is close to the threshold of positivity. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).