Lau 2020b.

Study characteristics
Patient Sampling	Purpose: Two‐group study to estimate sensitivity and specificity for diagnosis of acute and convalescent‐phase Covid Design: [1] PCR‐positive Covid‐19 cases (n = 338) of suspected or confirmed SARS‐CoV‐2 infection [2] Healthy healthcare workers (laboratory staff and frontline healthcare workers) with no suspicion for Covid‐19 (n = 294) [3] Samples positive for other antibodies including: dengue (n = 46), anti‐HCV (n = 3), HBsAg (n = 8), anti‐HBc IgM (n = 2), rheumatoid factor (n = 5). Recruitment: Unclear Prospective or retrospective: Retrospective Sample size: 696 (338) Further detail: No more details available
Patient characteristics and setting	Setting: Unclear (includes hospital inpatients but unclear if outpatients also included) Location: Changi General Hospital Country: Singapore Dates: April to May 2020 Symptoms and severity: Not stated Demographics: Not stated Exposure history: Not stated Non‐Covid group 1: Healthy healthcare workers Source: Volunteer staff in the same hospital, unclear if same time period as the cases or pre‐pandemic; described as 'coronavirus disease 2019 (COVID‐19)‐naive samples' Characteristics: No suspicion of Covid‐19 Non‐Covid group 2: Other disease serum samples Source: Unclear timing and source Characteristics: Not stated
Index tests	Test name: Architect SARS‐CoV‐2 IgG assay Manufacturer: Abbott Laboratories, USA Antibody: IgG Antigen target: Undisclosed epitope on the viral nucleocapsid Evaluation setting: Laboratory Test method: Qualitative chemiluminescent microparticle immunoassay (CLIA) Timing of samples: Timing was post‐PCR+ve: 0‐7 days: 155/266 (58%) 7‐14 days: 57/266 (21%) 14‐21 days: 22/266 (8%) >= 21 days: 32/266 (12%) Samples used: Serum Test operator: Unclear Definition of test positivity: The manufacturer cut‐off index (COI) of 1.4 was adopted to identify positivity Blinding reported: Unclear Threshold predefined: Yes, as per manufacturer
Target condition and reference standard(s)	Reference standard: Duplex real‐time PCR targeting E and N gene Samples used: Not stated Timing of reference standard: Not stated Blinded to index test: Yes, as occurred before index test Incorporated index test: No Definition of non‐COVID cases: [2] None; unclear if pre‐pandemic [3] Unclear Samples used: Unclear Timing of reference standard: Unclear Blinded to index test: Unclear Incorporated index test: No
Flow and timing	Time interval between index and reference tests: Unclear All patients received same reference standard: No Missing data: Excluded 5/338 with unknown PCR status, 10 PCR‐negative, and 57 inpatients "not initially suspected of having COVID‐19 but subsequently tested positive for SARS‐CoV‐2 PCR" Uninterpretable results: None reported Indeterminate results: None reported Unit of analysis: Patients Unclear; referred to 'cases' and samples
Comparative
Notes	Funding: No funding statement reported Publication status: Preprint Source: MedRxiv Author COI: All authors had nothing to disclose.
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?	Unclear
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?	Yes
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?			Low concern
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?	Unclear
Were results presented per patient?	Yes
Could the patient flow have introduced bias?		High risk