Table 4. IgM reactivity to N protein and S protein in subjects with suspected false-negative IgM results.

Sample	S+N protein (AU/mL)	N protein (AU/mL)	S protein (AU/mL)
1	53.76	97.17	0.34
2	16.43	18.14	0.96
3	13.03	10.51	0.34
4	17.94	29.31	0.69
5	110.11	130.57	1.15

We investigated the IgM reactivity to N protein and S protein in five subjects with suspected false-negative IgM results. N or S protein means the value measured using magnetic beads coated with either antigen, respectively.