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OBJECTIVE The EBNA1 IgA antibody level of normal and NPC subjects in a high incidence area were analyzed for new diagnostic criteria to improve diagnosis.METHODS A total of 780 normal and 104 NPC sera were tested for EBNA1 IgA antibody levels by ELISA. Two diagnostic criteria were obtained from sensitivity and specificity data: 1) lower equivocal limit (rOD=1.10) where sensitivity = 95%; and 2) upper equivocal limit (rOD=1.85) where specificity = 95%.RESULTS The range and distribution of EBNA1 IgA antibody levels are broad with those of normal subjects (0.093-4.726, mean = 0.850 ± 0.637) overlapping those from NPC subjects (0.235-3.721, mean = 2.241± 0.875). However, NPC subjects did exhibit significantly higher antibody levels (t = 18.5,P＜0.001). Based on the diagnostic criteria, 3 diagnostic categories were escentage of NPC subjects falling into these 3 diagnostic categories were 75.13%, 17.44% and 7.44%, respectively and of normal subjects, 4.81%,17.31%, 77.88% respectively.CONCLUSION Due to the broad distribution and overlapping of antibody levels between normal and NPC subjects in a high incidence area, it is important to have diagnostic criteria that will categorize those with equivocal results to minimize misdiagnosis. The 3 diagnostic categories established in this study will enhance detection and help physicians in their clinical diagnosis.