TABLE 1.

Sample characterization by RfB prior to dispatch to the participants^a

Sample no.	EQA scheme no.	VNT titer	IgG characteristics						IgA characteristics			IgM characteristics		Medical report
			Target value	Laboratory 1				Laboratory 2	Target value	Laboratory 1	Laboratory 2	Target value	Laboratory 1
				Roche Elecsys anti-N (COI)c	Roche Elecsys anti-S (U/ml)d	Euroimmun anti-S(ratio)e	Epitope anti-N(ratio)f	Euroimmun anti-S(ratio)g		Euroimmun anti-S(ratio)h	Euroimmun anti-S(ratio)i		Epitope anti-N(ratio)j
1	1	1:40b	Positive	2.86	NA	1.41	0.41	1.1	ND	ND	0.97	Negative	0.11	SARS-CoV-2 positive-tested, symptomatic patient. Blood sampling 34 days after symptom onset.
2	1	1:10	Positive	34.64	NA	2.91	0.44	2.11	ND	ND	0.22	Negative	0.12	SARS-CoV-2 positive-tested, asymptomatic patient. Blood sampling 39 days after symptom onset.
3	1	<1:10	Negative	0.07	NA	0.49	0.19	0.32	ND	ND	0.36	Negative	0.13	SARS-CoV-2-negative patient.
4	1	<1:10	Negative	0.48	NA	0.75	0.22	0.51	ND	ND	0.49	Negative	0.13	SARS-CoV-2-positive-tested, oligosymptomatic patient. Blood sampling 26 days after symptom onset.
5	2	<1:10	Positive	5.01	NA	0.84	ND	0.84	Negative	0.7	0.54	ND	ND	SARS-CoV-2-postitive-tested, symptomatic patient. Blood sampling 37 days after symptom onset.
6	2	<1:10	Negative	0.05	NA	0.35	ND	0.2	Negative	0.57	0.27	ND	ND	SARS-CoV-2 negative patient.
7	2	<1:10	Negative	0.06	NA	0.48	ND	0.26	Negative	1.63	0.59	ND	ND	SARS-CoV-2 negative patient pool.
8	2	1:20	Positive	80.46	NA	5.75	ND	7.86	Positive	2.44	1.7	ND	ND	SARS-CoV-2-positive-tested, oligosymptomatic patient. Blood sampling 59 days after symptom onset.
9	3	1:20	Positive	15.02	53.05	2.36	ND	3.3	Positive	1.19	2.3	ND	ND	SARS-CoV-2 positive-tested, symptomatic patient. Blood sampling 85 days after symptom onset.
10	3	<1:10	Positive	0.89	9.09	1.16	ND	1.36	Positive	1.89	2.84	ND	ND	SARS-CoV-2-positive-tested, oligosymptomatic patient. Blood sampling 32 days after symptom onset.
11	3	1:40	Positive	12.23	15.49	6.05	ND	9.56	Positive	3.69	5.37	ND	ND	SARS-CoV-2-positive-tested, oligosymptomatic patient. Blood sampling 47 days after symptom onset.
12	3	<1:10	Negative	0.06	0.4	0.40	ND	0.28	Negative	0.28	0.24	ND	ND	SARS-CoV-2-negative patient pool.

^aEQA, external quality assessment; VNT, virus neutralization test; ND, not determined; anti-N, antinucleocapsid; anti-S, antispike; NA, not applicable.

^bDetermined in undiluted sample.

^c<1.0 negative; ≥1.0 positive.

^d<0.8 negative; ≥0.8 positive.

^e≥1.1 positive, ≥0.8 to <1.1 borderline; <0.8 negative.

^f≥0.334 positive, <0.334 to >0.274 borderline; ≤0.274 negative.

^g≥1.1 positive, ≥0.8 to <1.1 borderline; <0.8 negative.

^h≥1.1 positive, ≥0.8 to <1.1 borderline; <0.8 negative.

ⁱ≥1.1 positive, ≥0.8 to <1.1 borderline; <0.8 negative.

^j≥0.216 positive, <0.216 to >0.177 borderline; ≤0.177 negative.