ObjectiveTo evaluate the reliability and clinical value for detecting paraquat (PQ)concentration in serum by spectrometry. MethodsThe determinations of wave length for detecting serum PQ concentration by ordinary spectrometry and second-derivative spectrometry were carried out. When the second-derivative spectrometry was used for detecting PQ in serum, the linear range and precision for PQ concentration were well defined. The results of serum PQ concentration determined by second-derivative spectrometry and by HPLC (high performance liquid chromatography) were compared in 8 patient with PQ poisoning. A total of 21 patients with acute poisoning after PQ ingestion over 4 hours admitted from October 2008 through September 2010 were retrospectively studied. Patients were divided into two groups as per the serum concentrations more than 1.8 μg/mL or less than that by second-derivative spectrometry on the day of admission. The severity of clinical manifestations between two groups was analyzed with t-test or Fisher's exact probabilities analysis. Results ( 1 ) The absorption peak of 257 nm could not be found by using ordinary spectrometry to detect the PQ concentration in serum. (2) The calibration curve in the 0. 4 ～ 8.0μg/mL range for detecting PQ concentration by second-derivative spectrometry observed the Beer's law (r =0. 996) . The average retrieval rate of PQ was within the range of 95.0％～ 99. 5％ with relative standard deviation (RSD) within 1.35％～ 5.41％ ( n = 6), and the lowest detection limit was 0. 05μg/mL. (3) The results of PQ concentrations from 8 patients with PQ poisoning detected by second-derivative spectrometry were consistent with those of the quantitative determinations by HPLC ( r = 0. 995,P＜0. 01 ) . (4) The survival rate of patients with serum PQ concentration more than 1.8 μg/mL was 22. 2％ ,and the incidences of acidosis, oligouria and pneumomediastium in these patients were 55.6％,55. 6％ and 77.8％, respectively. These clinical manifestations were significantly different from those in patients with serum PQ concentration less than 1.8 μg/mL ( P ＜ 0. 05 ) . Conclusions ( 1 ) It was inappropriate to take 257 nm as the determination wave length for detecting serum PQ concentration by ordinary spectrometry. (2) The method of second-derivative spectrometry was reliable for detecting serum PQ concentration. (3) Serum PQ concentration detected by second derivative spectrometry could be used to predict the severity of clinical manifestations of patients with PQ poisoning and was an important predictive factor for poor prognosis if the serum PQ concentration more than 1.8 μg/mL after PQ ingestion over 4 hours.