Du 2021.

Study characteristics
Patient Sampling	Purpose: Diagnosis of current acute‐phase infection Design: Multi‐group study to estimate sensitivity and specificity [1] Confirmed COVID‐19 (n = 107) [2] Pre‐pandemic non‐COVID (n = 226) [2a] Healthy donor samples (n = 138) [2b] Cross‐reaction challenge samples (n = 88) Recruitment: [1] COVID‐19 patient serum samples were acquired from ProMedDx (Norton, MA) and University of California and VA Healthcare System. [2a] Healthy donor EDTA K2 plasma samples were purchased from Golden West Biosolutions (Temecula, CA) in 2019 prior to the outbreak of COVID‐19. COVID‐19 negative EDTA K2 plasma samples were also obtained from University of Florida Department of Radiation Oncology in 2017. Healthy donor serum samples were purchased from Innovative Research, LLC (Plymouth, MN). [2b] Patient serum samples positive for IgG to HBV/HCV/HIV/RSV were purchased from Antibody Systems, Inc (Hurst, TX). Patient serum samples positive for IgG to HAV/CMV/EBV/Rubella/Influenza B were purchased from ProMedDx (Norton, MA). Patient serum samples positive for IgG to Influenza A were purchased from Dx Biosamples, LLC (San Diego, CA). Prospective or retrospective: Retrospective Sample size: 333 (107) of which 252 (26) with eligible time splits Further detail: Not stated
Patient characteristics and setting	Setting: Not stated Location: ProMedDx (Norton, MA) and University of California and VA Healthcare System Country: USA Dates: Not stated Symptoms and severity: Not stated Demographics: Not stated Exposure history: Not stated Non‐Covid group 1: [2] Pre‐pandemic healthy or cross‐reactivity Source: [2a] Healthy donor EDTA K2 plasma samples were purchased from Golden West Biosolutions (Temecula, CA) in 2019 prior to the outbreak of COVID‐19. COVID‐19 negative EDTA K2 plasma samples were also obtained from University of Florida Department of Radiation Oncology in 2017. Healthy donor serum samples were purchased from Innovative Research, LLC (Plymouth, MN). [2b] Patient serum samples positive for IgG to HBV/HCV/HIV/RSV were purchased from Antibody Systems, Inc (Hurst, TX). Patient serum samples positive for IgG to HAV/CMV/EBV/Rubella/Influenza B were purchased from ProMedDx (Norton, MA). Patient serum samples positive for IgG to Influenza A were purchased from Dx Biosamples, LLC (San Diego, CA). Characteristics: [2a] Healthy donors (n = 138) [2b] Cross‐reactivity (n = 88) HIV n = 4 Hepatitis A virus n = 7 Hepatitis B virus n = 4 Hepatitis C virus n = 4 Respiratory syncytial virus n = 5 Influenza A n = 5 Influenza B n = 13 Cytomegalovirus n = 16 Epstein‐Barr virus n = 13 Rubella n = 17 Non‐Covid group 2: NA Source: NA Characteristics: NA
Index tests	Test name: QuantiVirus™ anti‐SARS‐CoV‐2 IgG test Manufacturer: DiaCarta Inc, 2600 Hilltop Dr. Richmond, CA 94806, United States Antibody: IgG Antigen target: spike‐protein 1 (S1) RBD Evaluation setting: Laboratory test performed in lab Test method: Fluorescence immunoassay Phycoerythrin fluorescence of each well in a 96‐well microplate was measured on Luminex 200 or MAGPIX® instrument for Median Fluorescence Intensity (MFI) Timing of samples: 0‐7 days pso: 13/107 8‐14 days pso: 13/107 > 14 days pso: 81/107 Samples used: [1] Serum [2a] Serum and plasma [2b] Serum Test operator: Lab personnel Definition of test positivity: Median Fluorescence Intensity (MFI). Interpretation of the testing results was performed by calculating the MFI ratio of each sample to the average MFI of two blank wells. Blinding reported: Not stated Threshold predefined: Not stated
Target condition and reference standard(s)	Reference standard: RT‐PCR Samples used: Not stated Timing of reference standard: Not stated Blinded to index test: yes, prior index test Incorporated index test: no Definition of non‐COVID cases: [2] Pre‐pandemic (time not stated for all sources) Samples used: [2] Pre‐pandemic Timing of reference standard: [2] Pre‐pandemic Blinded to index test: yes, prior index test Incorporated index test: No
Flow and timing	Time interval between index and reference tests: Not stated All patients received same reference standard: No Missing data: Not stated Uninterpretable results: Not stated Indeterminate results: Not stated Unit of analysis: Not stated
Comparative
Notes	Funding: Not stated Publication status: Published paper Source: Journal of Virological Methods Author COI: The authors reported no declarations of interest.
Methodological quality
Item	Authors' judgement	Risk of bias	Applicability concerns
DOMAIN 1: Patient Selection
Was a consecutive or random sample of patients enrolled?	Unclear
Was a case‐control design avoided?	No
Did the study avoid inappropriate exclusions?	Unclear
Did the study avoid inappropriate inclusions?	No
Could the selection of patients have introduced bias?		High risk
Are there concerns that the included patients and setting do not match the review question?			High
DOMAIN 2: Index Test (All tests)
DOMAIN 2: Index Test (Antibody tests)
Were the index test results interpreted without knowledge of the results of the reference standard?	Unclear
If a threshold was used, was it pre‐specified?	Unclear
Could the conduct or interpretation of the index test have introduced bias?		Unclear risk
Are there concerns that the index test, its conduct, or interpretation differ from the review question?			Unclear
DOMAIN 3: Reference Standard
Is the reference standards likely to correctly classify the target condition?	Unclear
Were the reference standard results interpreted without knowledge of the results of the index tests?	Yes
The reference standard does not incorporate the index test	Yes
Could the reference standard, its conduct, or its interpretation have introduced bias?		Unclear risk
Are there concerns that the target condition as defined by the reference standard does not match the question?			High
DOMAIN 4: Flow and Timing
Was there an appropriate interval between index test and reference standard?	Unclear
Did all patients receive the same reference standard?	No
Were all patients included in the analysis?	Unclear
Did all participants receive a reference standard?	Yes
Were results presented per patient?	Unclear
Could the patient flow have introduced bias?		High risk