Lau 2020a.

Study characteristics
Patient Sampling	Purpose: Diagnosis of acute and convalescent‐phase Covid‐19 Design: [1] PCR‐confirmed Covid‐19 cases (n = 280 patients providing 415 samples) [2] Healthy heathcare worker controls (n = 597); 315 with annual southern hemisphere influenza vaccination 4 weeks prior [3] Antibody positive for different diseases: dengue (n = 74), hepatitis C (n = 3), hepatitis B (n = 12), syphilis (n = 1), antinuclear antibody (n = 16) double‐stranded DNA antibody (n = 4), rheumatoid factor (n = 7) Recruitment: Not stated Prospective or retrospective: Retrospective Sample size: 994 (280) Further detail: No more details available
Patient characteristics and setting	Setting: Unclear (hospital patients but unclear if inpatient or outpatient) Location: Changi General Hospital, Khoo Teck Puat Hospital, Sengkang General Hospital Country: Singapore Dates: April to June 2020 Symptoms and severity: Not stated Demographics: Unclear Not stated Exposure history: Unclear 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: From ambulatory subjects with no suspicion for Covid‐19 or acute respiratory illness; unclear timing Characteristics: Not stated
Index tests	Test name: ELECSYS anti‐SARS‐CoV‐2 assay. Manufacturer: Roche Diagnostics, Switzerland Antibody: Unclear Not stated; appeared to be total Ab (not specified in IFU either) Antigen target: Biotinylated SARS CoV‐2 specific recombinant antigens and SARS‐CoV‐2 specific recombinant antigens labelled with ruthenium Evaluation setting: Laboratory Test method: CLIA (Sandwich immunoassay) Timing of samples: Timing reported was post‐PCR+ve 0‐7 days: 189/349 (54%) 7‐13 days: 90/349 (26%) 14‐20 days: 34/349 (10%) >= 21 days: 36/349 (10%) Samples used: Serum Test operator: Unclear Definition of test positivity: An anti‐SARS‐CoV‐2 index was derived with a reported cut‐off index (COI) of 1.0 for positivity. Blinding reported: Unclear Threshold predefined: Yes, as per manufacturer
Target condition and reference standard(s)	Reference standard: PCR; no further details 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: None reported; NB methods stated 415 samples included from 280 patients but results reported total of 419 samples Uninterpretable results: None reported Indeterminate results: None reported Unit of analysis: Samples
Comparative
Notes	Funding: Nothing stated Publication status: Pre‐print (not peer reviewed) 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?	Unclear
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?	No
Did all participants receive a reference standard?	No
Were results presented per patient?	Unclear
Could the patient flow have introduced bias?		High risk