Jaaskelainen 2020.

Study characteristics
Patient Sampling	Purpose: Diagnosis of acute‐phase infection Design: Multi‐group study to assess sensitivity and specificity: [1] COVID‐19‐positive patients (n = 47) [2] Non‐COVID‐19 (n = 37 patients) [3] Probable COVID‐19 patients (according to WHO definition) who had tested negative for SARS‐CoV‐2 by NAT (n = 13) Recruitment: Unclear Prospective or retrospective: Restrospective Sample size: 97(47) but 84(37) could be used for our review Further detail: Inclusion: [1] Diagnosed with COVID‐19 by RT‐PCR [2] Patients diagnosed with seasonal human coronaviruses or other respiratory viruses or viral infections, either pre‐pandemic or RT‐PCR‐negative for SARS‐CoV‐2 Excluded: [1][2] Not stated
Patient characteristics and setting	Setting: Hospital Location: Helsinki University Hospital Country: Finland Dates: Not stated Symptoms and severity: Mild symptoms 9/37, moderate symptoms 15/37, severe symptoms 13/37 Demographics: [1] 23/40 (57.5%) males, median age 56 years (range: 24–77 years) Exposure history: Not stated Non‐Covid group 1: Non‐COVID‐19 patients Source: 2019‐2020, source not stated Characteristics: 15/37 (%) male, median age: 53 years (range: 5‐87 years). Patients diagnosed with seasonal human coronaviruses (OC43, NL63, 229E) or other respiratory viruses by nucleic acid tests (n = 11) and samples from patients who had been diagnosed as having adenovirus, enterovirus, influenza A, influenza B, parainfluenza, or respiratory syncytial (RSV) virus infections, through routine IgG antibody testing in 2019 (n = 26). Samples from 2019 were assumed to be from SARS‐CoV‐2 negative patients, while samples obtained in 2020 were from patients who had been tested for SARS‐CoV‐2 nucleic acid and found negative.
Index tests	Test name: SARS‐CoV‐2 IgG and IgA ELISA Manufacturer: Euroimmun, Lübeck, Germany Antibody: IgG, IgA Antigen target: S1‐protein Evaluation setting: Laboratory Test method: ELISA Timing of samples: 1‐23 days pso Samples used: Serum Test operator: Not stated Definition of test positivity: ratio < 0.8 was considered negative, ≥ 0.8 and < 1.1 inconclusive and ≥ 1.1 positive Blinding reported: Not stated Threshold predefined: Probably (commercial test), ratio < 0.8 was considered negative, ≥ 0.8 and < 1.1 inconclusive and ≥ 1.1 positive.
Target condition and reference standard(s)	Reference standard: RT‐PCR Samples used: nasopharyngeal swabs Timing of reference standard: not stated Blinded to index test: yes, prior Incorporated index test: No Definition of non‐COVID cases: Pre‐pandemic or RT‐PCR Samples used: Nasopharyngeal swabs Timing of reference standard: Not stated Blinded to index test: Yes, prior Incorporated index test: no
Flow and timing	Time interval between index and reference tests: 0‐14 days All patients received same reference standard: no Missing data: [1] One patient with a single sample taken before symptom onset was not further investigated. Uninterpretable results: not stated Indeterminate results: Indeterminate results classed as positive for analysis Unit of analysis: samples
Comparative
Notes	Funding: Not stated Publication status: Rapid Communication Source: Euro Surveillance Author COI: COI declared: None declared
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?	Yes
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?		Low 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?	No
Could the patient flow have introduced bias?		High risk