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Severe acute respiratory syndrome (SARS) has raised a global alert since March 2003. After its causative agent, SARS-associated coronavirus (SARS-CoV), was confirmed, laboratory methods, including virus isolation, reverse transcriptase–polymerase chain reaction (RT-PCR), and serologic methods, have been quickly developed. In this study, we evaluated four serologic tests ( neutralization test, enzyme-linked immunosorbent assay [ELISA], immunofluorescent assay [IFA], and immunochromatographic test [ICT]) for detecting antibodies to SARS-CoV in sera of 537 probable SARS case-patients with correlation to the RT-PCR . With the neutralization test as a reference method, the sensitivity, specificity, positive predictive value, and negative predictive value were 98.2%, 98.7%, 98.7%, and 98.4% for ELISA; 99.1%, 87.8%, 88.1% and 99.1% for IFA; 33.6%, 98.2%, 95.7%, and 56.1% for ICT, respectively. We also compared the recombinant-based western blot with the whole virus–based IFA and ELISA; the data showed a high correlation between these methods, with an overall agreement of >90%. Our results provide a systematic analysis of serologic and molecular methods for evaluating SARS-CoV infection.

Severe acute respiratory syndrome (SARS)‐CoV is a newly emerging virus that causes SARS with high mortality rate in infected people. To study the humoral responses against SARS‐CoV, we evaluated nucleocapsid (N) and spike (S) proteins‐specific antibodies in patients' sera by Western blotting and enzyme‐linked immunosorbent assay (ELISA). Recombinant N and S proteins of SARS‐CoV were purified from transformed E. coli. Serum specimens from 40 SARS‐CoV‐infected patients in the convalescent phase were analyzed by Western blotting using the purified antigens. Serial serum specimens from 12 RT‐PCR‐confirmed SARS patients were assayed by ELISA using the recombinant N protein as coated antigen. By Western blotting, 97.5% of the SARS patients were positive for N protein‐specific antibodies whereas only 47.5% of the samples were positive for S protein‐specific antibodies. Using N protein‐based ELISA, 10 out of the 12 patients were positive for N protein‐specific antibodies and 6 of them showed seroconversion at mean of 16 days after onset of fever. Immunoblotting was useful for detecting the humoral immune response after SARS‐CoV infection. Antibodies against SARS‐CoV N protein appear at the early stage of infection, therefore, N protein‐based ELISA could serve as a simple, sensitive, and specific test for diagnosing SARS‐CoV infection. J. Med. Virol. 73:338–346, 2004. © 2004 Wiley‐Liss, Inc.