Haljasmagi 2020.

Study characteristics
Patient Sampling	Purpose: Detection of acute and convalescent‐phase SARS‐CoV‐2 antibodies Design: Two‐group study estimating sensitivity and specificity: [1] PCR‐confirmed hospitalised Covid‐19 patients (n = 26) [2] Healthy controls without recent infection or Covid‐19 symptoms (fever or cough) for last month) (n = 26) Recruitment: Unclear Prospective or retrospective: Unclear Sample size: 52 (26) Further detail: No more details available
Patient characteristics and setting	Setting: Hospital inpatient Location: Tartu University Hospital Country: Estonia Dates: Not stated Symptoms and severity: Not described Demographics: Median age 62 y (range 33‐91 y); 18, 51% male [calculated from Suppl Tabl 1] Exposure history: Not described Non‐Covid group 1: Contemporaneous apparently healthy controls Source: Unclear Characteristics: Without recent infection or Covid‐19 symptoms (fever or cough) for previous month; age range 23‐54 years No further details
Index tests	Test name: Anti‐SARS‐CoV‐2 IgG ELISA Manufacturer: Euroimmun, Germany Antibody: IgG Antigen target: S1 Evaluation setting: Laboratory, laboratory Test method: ELISA Timing of samples: median 16 days (range 8 to 37 d) Day 8‐14 after infection: 9/26 (35%) Day 15‐21 after infection: 11/26 (42%) Day 22+ after infection: 6/26 (23%) Samples used: Plasma Test operator: Unclear Definition of test positivity: "According to the manufacturer’s recommendations, a ratio < 0.8 is considered negative, ≥ 0.8 and < 1.1 borderline, and ≥ 1.1 positive." Blinding reported: Unclear Threshold predefined: Yes, as per manufacturer
Target condition and reference standard(s)	Reference standard: PCR; no further details Samples used: Unclear Timing of reference standard: Unclear Blinded to index test: Unclear Incorporated index test: No Definition of non‐COVID cases: Unclear, no SARS‐CoV‐2 testing reported Samples used: Unclear, possibly none 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 (controls had different reference) Missing data: Nothing mentioned Uninterpretable results: Nothing mentioned Indeterminate results: Nothing mentioned Unit of analysis: Patients
Comparative
Notes	Funding: "The study was supported by the Estonian Research Council grants [#] (PP) and [#] (K.K.)" Publication status: Published letter Source: European Journal of Immunology Author COI: The authors declared no commercial or financial conflict of interests.
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?	Unclear
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