Figure 1.

N protein detection in 420 serum samples from 317 patients with severe acute respiratory syndrome (SARS). Data represent the optical density at 450 nm (OD₄₅₀) of undiluted serum samples. To establish the normal range of the N protein–capture enzyme-linked immunosorbent assay, serum specimens from 400 healthy blood donors were analyzed. The mean OD₄₅₀ for these specimens, as determined by the assay, was 0.078, with a standard deviation of 0.023. The cutoff OD₄₅₀ of the assay was then calculated as follows: cutoff = mean of OD₄₅₀ from 400 normal serum + 5 x standard deviations = 0.19. Solid line represents cutoff value. The result was considered positive if a sample yielded OD₄₅₀ above the cutoff.