Liu 2020b [A].

Study characteristics
Patient Sampling	2‐group study to estimate sensitivity and specificity in acute and convalescent‐phase sera 1. RT‐PCR‐confirmed COVID‐19 cases (n = 214) 2. Healthy blood donors (n = 100) Retrospective design; recruitment method NR. No further detail
Patient characteristics and setting	[1] Inpatients at General Hospital of the Central Theater Command of the People’s Liberation Army (PLA), China (recruitment dates 18 January‐26 February). Exposure history and participant characteristics not described [2] Healthy blood donors; not further described
Index tests	2 Ab tests, blinding NR; this entry (Liu 2020b [A]) refers to test [A] in the list below Laboratory‐based evaluations of ELISA assays measuring IgM and IgG using serum samples: A. rN‐based ELISA (Lizhu, Zhuhai, China), using recombinant N‐protein B. rS‐based ELISA (Hotgen, Beijing, China), using receptor‐binding domain of the recombinant S polypeptide (rS) Test thresholds: A. cut‐off calculated by summing 0.100 (IgM) or 0.130 (IgG) and the average A450 of negative control replicates. When A450 < cut‐off value, the test was considered negative, and when A450 was ≥ cut‐off value, the test was considered positive. B. cut‐off values (IgM and IgG) calculated by summing 0.250 and the average A450 of negative control replicates. When A450 < cut‐off value, the test was considered negative, and when A450 was ≥ cut‐off value, the test was considered positive. Samples acquired 0‐5 d 22, 10%; 6‐10 d 38, 18%; 11‐15 d 54, 25%; 16‐20 d 55, 26%; ≥ 21 d 45, 21% (32/45 were d 21‐30). Person applying the test not described
Target condition and reference standard(s)	[1] RT‐PCR (no further detail), using pharyngeal swabs samples. Positivity threshold NR. Samples acquired at a median of 15 d pso (range 0–55 days) 2. Healthy blood donors; no description of timing of serum sample collection
Flow and timing	Sampling for index and reference for cases was conducted within same time frame. No missing data, uninterpretable or indeterminate results described Basis for analysis: participants. Included a single sample per participant with results disaggregated by time pso, but different participants contributed data to each time period
Comparative
Notes	Supported by the National Natural Science Foundation, the China Postdoctoral Science Foundation (2019M664008), and the Wuhan Young and Middle‐aged Medical Backbone Talents Training Project (Wuweitong [2019] 87th266) Accepted manuscript (Journal of Clinical Microbiology) No conflicts of interest 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?	No
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?		High 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?	Yes
Could the patient flow have introduced bias?		High risk