Carta 2020.

Study characteristics
Patient Sampling	Purpose: Diagnosis of current convalescent‐phase infection Design: Single‐group study to assess sensitivity and specificity (n = 65) [1] Covid positive residents (n = 54) [2] Covid negative residents (n = 11) Recruitment: All residents in a long‐term care facility Prospective or retrospective: Prospective Sample size: 65 (54) Further detail: Inclusion: All the guests (symptomatic and asymptomatic) of a long‐term care facility. [1] Residents who tested positive for SARS‐CoV‐2 infection on RT‐PCR during any of three tests [2] Residents who tested negative for SARS‐CoV‐2 infection on all of three RT‐PCR tests Exclusion: [1] [2] No exclusion criteria; all residents included
Patient characteristics and setting	Setting: Long‐term care facility, all convalescent Location: Vicenza district Country: Italy Dates: PCR test performed between March 29 and April 22, 2020. Follow‐up for 2 months after outbreak Symptoms and severity: Symptomatic and asymptomatic, including 11 cases of fatal infection Demographics: 52/65 female, average age 82 years (range 56‐97 years) 26 not self‐sufficient Exposure history: Not stated Non‐Covid group 1: NA
Index tests	Test name: [A] MAGLUMI 2019‐nCoV IgG and IgM Manufacturer: [A] [B] Shenzen New Industries Biomedical Engineering Co., SNIBE Diagnostic, Shenzen, PR China Antibody: [A] IgG/IgM Antigen target: [A] spike‐protein and nucleocapsid region Evaluation setting: Laboratory used in laboratory Test method: [A] CLIA Timing of samples: Day 32 (28–36) and 49 (47–50) post‐PCR +ve Samples used: Serum Test operator: Not stated, possibly Medicina di Laboratorio, AULSS 8 Berica, Viale Rodolfi, Vicenza, Italy Definition of test positivity: [A] IgG antibodies were considered negative < 0.90 AU/mL, grey‐zone 0.90‐1.10 AU/mL and positive >= 1.10 AU/mL [B] IgM antibodies were considered positive >= 1.00 AU/mL, negative < 1.00 AU/mL. Blinding reported: Not clear Threshold predefined: Yes, according to manufacturer
Target condition and reference standard(s)	Reference standard: RT‐PCR on Cobas 6800 RT‐PCR System (Roche Diagnostics GmbH, Mannheim, Germany) When two gene targets were found both positive, or even if only one target was found, but the patient had characteristic symptoms, the test was considered positive. Samples used: Oropharyngeal and nasopharyngeal swabs Timing of reference standard: Start of outbreak at long‐term care facility then on days 20, 32 and 49 Blinded to index test: Yes, prior Incorporated index test: No Definition of non‐COVID cases: RT‐PCR on Cobas 6800 RT‐PCR System (Roche Diagnostics GmbH, Mannheim, Germany) When two gene targets were found both positive, or even if only one target was found, but the patient had characteristic symptoms, the test was considered positive. Samples used: Oropharyngeal and nasopharyngeal swabs Timing of reference standard: Start of outbreak at long‐term care facility then on days 20 and 32. Blinded to index test: Yes Incorporated index test: No
Flow and timing	Time interval between index and reference tests: 32 (28–36) and 49 (47–50) days All patients received same reference standard: Yes Missing data: Among 65 residents, 54 tested positive for COVID‐19 on the first swab but 11 of these patients subsequently died. Uninterpretable results: Not stated Indeterminate results: Grey zone for IgG antibody detection results, 0.90‐1.10 AU/mL, but no indeterminate results reported Unit of analysis: Samples (one sample on day 32 and one sample on day 49)
Comparative
Notes	Funding: None declared Publication status: Published paper Source: De Gruyter Diagnosis Author COI: Authors stated no conflict 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?	Yes
Was a case‐control design avoided?	Yes
Did the study avoid inappropriate exclusions?	Yes
Did the study avoid inappropriate inclusions?	Yes
Could the selection of patients have introduced bias?		Low risk
Are there concerns that the included patients and setting do not match the review question?			Low concern
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?	Yes
Were all patients included in the analysis?	Yes
Did all participants receive a reference standard?	No
Were results presented per patient?	No
Could the patient flow have introduced bias?		High risk