key: cord-293544-nemw29r7 authors: Valdivia, Arantxa; Torres, Ignacio; Huntley, Dixie; Alcaraz, María J.; Albert, Eliseo; Colomina, Javier; Ferrer, Josep; Carratalá, Arturo; Navarro, David title: Qualitative assessment of SARS‐CoV‐2‐specific antibody avidity by lateral flow immunochromatographic IgG/IgM antibody assay date: 2020-08-02 journal: J Med Virol DOI: 10.1002/jmv.26344 sha: doc_id: 293544 cord_uid: nemw29r7 Knowledge of the precise timing of SARS‐CoV‐2 infection may be of clinical and epidemiological relevance. The presence of low‐avidity IgGs has conventionally been considered an indicator of recent infection. Here, we carried out qualitative assessment of SARS‐CoV‐2‐specific antibody avidity using an urea (6M) dissociation test performed on a lateral flow immunochromatographic IgG/IgM device. We included a total of 76 serum specimens collected from 57 COVID‐19 patients, of which 39 tested positive for both IgG and IgM and 37 only for IgG. Sera losing IgG reactivity after urea treatment (n = 28) were drawn significantly earlier (P = .04) after onset of symptoms than those which preserved it (n = 48). This assay may be helpful to estimate the time of acquisition of infection in patients with mild to severe COVID‐19. Detection of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) RNA in respiratory tract specimens by reversetranscription-based polymerase chain reaction (RT-PCR) assays is the mainstay of corona virus 2019 (COVID-19) diagnosis. 1 However, a nonnegligible fraction of COVID-19 patients test negative by RT-PCR on initial or consecutive upper respiratory tract specimens, due to a number of nonmutually exclusive preanalytical or analytical factors. 2 Although serology testing is mainly aimed at identifying individuals who have previously been exposed to SARS-CoV-2, it may also aid in diagnosis of ongoing COVID-19, particularly in RT-PCR negative patients who present at relatively late times after infection. 3 Knowledge of the precise timing of infection may be of clinical and epidemiological relevance as viral shedding in the upper respiratory tract seems to continue up to 7 to 9 days after onset of symptoms in patients presenting with mild or moderate COVID-19. 4 Commercially-available SARS-CoV-2 RT-PCR assays used in the current study were detailed in previous publications. 12, 13 2.3 | SARS-CoV-2 antibody avidity assay Absence of discernible lines was recorded as negative. Complete disappearance of reactive lines after urea treatment was interpreted as presence of low-avidity antibodies, whereas their persistence was taken to indicate high-avidity antibody presence. The Mann-Whitney U-test was used for comparison of medians. Table 1 . Based upon the assumption that viable SARS-CoV-2 can be detected in URT specimens up to 9 days after symptoms onset, 4-6 we grouped sera into two categories according to whether they were drawn either before (n = 10) or after day 9 (n = 66) since symptoms onset. Low-avidity IgGs were detected in 8 out of the 10 former sera, and in 21 of the 66 latter sera (P = .01). Accordingly, the positive predictive value of the assay (IgG band) for correctly categorizing SARS-CoV-infection as an early one (<10 days after the onset of symptoms) was 80%, whereas the negative predictive value was 68%. Both predictive values were not adjusted to SARS-CoV-2 prevalence. At least two consecutive sera were available from 15 patients ( Table 2 ). Acquisition of high-avidity IgG antibodies in our system was The authors are grateful to all personnel who work at Clinic University Hospital, in particular to staff at the Microbiology laboratory for their commitment to the fight against COVID-19. The authors declare no conflicts of interest. T A B L E 2 Qualitative assessment of SARS-CoV-2-specific antibody avidity in serial serum samples from patients with COVID Laboratory testing strategy recommendations for COVID-19: interim guidance False-negative results of initial RT-PCR assays for Covid-19: a systematic review SARS-CoV-2 antibody testing-questions to be asked Temporal profiles of viral load in posterior oropharyngeal saliva samples and serum antibody responses during infection by SARS-CoV-2: an observational cohort study Virological assessment of hospitalized patients with COVID-2019 Predicting infectious SARS-CoV-2 from diagnostic samples The distribution and functions of immunoglobulin isotypes. Immunobiology: The immune system in health and disease Antibody responses to SARS-CoV-2 in patients with COVID-19 Immunoglobulin somatic hypermutation Clinical utility of avidity assays A method to prevent SARS-CoV-2 IgM false positives in gold immunochromatography and enzyme-linked immunosorbent assays Co-detection of respiratory pathogens in patients hospitalized with Coronavirus viral disease-2019 pneumonia Pooling of nasopharyngeal swab specimens for SARS-CoV-2 detection by RT-PCR Qualitative assessment of SARS-CoV-2-specific antibody avidity by lateral flow immunochromatographic IgG/IgM antibody assay