Graham 2021.

Study characteristics
Patient Sampling	Purpose: Diagnosis of prior infection (sero‐prevalence in nursing home residents) Design: Single‐group study to estimate sensitivity and specificity [1] Confirmed COVID patients (PCR+) (n = 94) [2] PCR‐ residents (n = 147) PCR‐ residents (n = 147) were not included in our review as they did not have an adequate reference standard (PCR tests performed too late or not correctly swabbed). Recruitment: Testing was performed as part of an outbreak investigation with Public Health England and verbal consent obtained from residents (or their relative/friend as appropriate) who had a RT‐PCR result available. All residents available and consenting to testing from 4 UK nursing homes Prospective or retrospective: Prospective Sample size: 241 (94) samples of which 94 (94) samples were eligible for our review Further detail: All residents of 4 UK Nursing Homes with rt‐PCR results available and informed consent [1] All rt‐PCR‐positive residents
Patient characteristics and setting	Setting: Convalescent (Nursing home residents) Location: 4 UK Nursing Homes (West London Nursing Homes) Country: UK Dates: June 2020 Symptoms and severity: Convalescent (around 2 months after outbreak) [Of 158 PCR+ residents, 43% had no identifiable symptoms in the preceding two‐week period. 35% of antibody‐positive residents (62 of 173) had been asymptomatic in the two‐week ascertainment window prior to PCR testing during the outbreak. Not stated for the 94 included COVID cases] Demographics: Not stated (high‐dependency nursing home residents) Exposure history: All nursing home residents
Index tests	Test name: Abbott Architect nucleocapsid IgG assay Manufacturer: Abbott Antibody: IgG Antigen target: N‐protein Evaluation setting: Lab test performed in lab Test method: Not stated Timing of samples: Not stated (convalescent, around 2 months after diagnosis) Samples used: Serum Test operator: Not stated (as part of an outbreak investigation with Public Health England) Definition of test positivity: Not stated (samples with binding ratios near to the cut‐off were confirmed on an in‐house receptor binding domain double antigen bridging assay to determine final status) Blinding reported: Not stated Threshold predefined: Not stated
Target condition and reference standard(s)	Reference standard: RT‐PCR testing for all residents, with re‐testing one week later in those testing negative Samples used: Oropharyngeal and nasal swabs Timing of reference standard: Not stated Blinded to index test: yes, prior index test Incorporated index test: no
Flow and timing	Time interval between index and reference tests: Around 2 months (PCR+ in April 2020, index test in June 2020) All patients received same reference standard: yes Missing data: yes (147 PCR‐ residents not included in our review) Uninterpretable results: Not stated Indeterminate results: Samples with binding ratios near to the cut‐off were confirmed on an in‐house receptor binding domain double antigen bridging assay to determine final status. Number not stated Unit of analysis: Patients
Comparative
Notes	Funding: UK DRI Centre for Care Research and Technology for funding the work Publication status: Published letter Source: Journal of Infection Author COI: Not stated
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?	No
Did the study avoid inappropriate exclusions?	Yes
Did the study avoid inappropriate inclusions?	Yes
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?	Unclear
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?	No
Did all participants receive a reference standard?	Yes
Were results presented per patient?	Yes
Could the patient flow have introduced bias?		High risk