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Notes: Background Our previous study reported that more than 50% of toluene diisocyanate (TDI)-induced asthma patients had persistent asthmatic symptoms even after complete avoidance. Although specific IgE (sIgE) has been detected in a portion of patients with TDI-asthma, a recent investigation suggests that the presence of serum specific IgG (sIgG), not sIgE, is more closely associated with positive bronchoprovocation test (BPT) results.Objective To evaluate the possible role of sIgE and sIgG in predicting long-term prognosis of TDI-asthma.Materials and methods Forty-one TDI-asthma patients whose diagnosis was confirmed by TDI-BPT, and 20 unexposed healthy controls were enrolled. Both sIgE and sIgG to TDI-human serum albumin (HSA) conjugate were detected by ELISA. All patients with persistent asthmatic symptoms took anti-asthmatic medications during the follow-up period (mean: 67.5 months) and were instructed to avoid exposure to TDI. Airway hyper-responsiveness to methacholine (AHM) was monitored every year during the study period. The patients were classified into three groups according to changing patterns of AHM and asthmatic symptoms as follows: group I, no improvement with persistent asthmatic symptoms (n = 12); group II, partial improvement with persistent asthmatic symptoms (n = 13); group III, in remission (n = 16).Results Favourable prognosis was associated with a mild degree of AHM at initial diagnosis (P 〈 0.05). Although there were no significant differences in the prevalence of sIgE antibody to TDI-HSA conjugate among the three groups (P 〉 0.05), prevalence of sIgG in group I tended to be higher than in group II (0.05 〈 P 〈 0.1). However, the levels of sIgG were significantly higher in group I than in group II (P = 0.05), whereas levels of sIgE were significantly higher in group II than in group I (P = 0.014). No significant differences were noted in exposure duration, sex, age, atopic status, and total IgE level among the three groups (P 〉 0.05).Conclusion This study confirmed that a favourable outcome is related to a mild degree of AHM and to low levels of sIgG to predict persistent asthmatic symptoms, it also suggested that the presence of high serum-specific IgE at initial diagnosis may represent a better prognosis.