Nucleocapsid protein of SARS coronavirus as a marker for the SARS serodiagnosis

Objective To study the temporal profile of the specific antibody against nucleocapsid(N) protein in patients with severe acute respiratory syndrome (SARS) and to evaluate the N protein used as diagnostic marker of SARS serodiagnosis. Methods A tolal of 500 sera, including 250 clinical documented, 188 clinical suspected and 62 clinically excluded SARS patients were analyzed by both the recombinant N protein and lysates of SARS-CoV based indirect enzyme-linked immunosorbent assay (ELISA). Results Among 250 sera from clinical documented patients with SARS, IgG responses to the recombinant N protein was detected in 4 of 116 (3.45%) at first week, 42 of 68 (61.76%) at second week , 18 of 22( 81.82% ) at third week and 44 of 44 (100%) from day 22 to 93 after the onset of symptoms respectively. In comparison, the positive rates of the IgG against SARS-CoV were 4.31% (5/116), 55.88% (38/68), 77.27% (17/22) and 100% (44/44) in the same four phases respectively. In 188 specimens from clinical suspected SARS patients, 3.19%(6/188) was IgG positive for N and 2.66% (5/188) for SARS-CoV respectively. Both IgG positive rate of the N and of SARS-CoV was 6.45% (4/62) in the specimens from clinical excluded SARS patients. The McNemar t test showed that the difference was not significant between the results of two methods for SARS serodiagnosis (χ2=0.180, P0.001). The result of the indirect ELISA based N-protein showed a 98.20% (491/500) agreement with that of the ELISA assay with SARS-CoV. Furthermore, a few samples of sera (1.88%, 14/745) from health donors may be cross-reacted with the recombinant N protein. Conclusion The recombinant N protein of SARS-CoV may be a cadidate of diagnostic marker for SARS serodiagnosis.