Comparison of the quantification of caffeine in human plasma by gas chromatography and ELISA

In the present study we evaluated the precision of the ELISA method to quan tify caffeine in human plasma and compared the results with those obtained by gas chromatography. A total of 58 samples were analyzed by gas chromatog raphy using a nitrogen-phosphorus detector and routine techniques. For the ELISA test, the samples were diluted to obtain a concentration correspondin g to 50% of the absorbance of the standard curve. To determine whether the proximity between the I-50 of the standard curve and that of the sample wou ld bring about a more precise result, the samples were divided into three b locks according to the criterion of difference, in modulus, of the I-50 Of the standard curve and of the I-50 Of the sample. The samples were classifi ed into three groups. The first was composed of 20 samples with I-50 up to 1.5 ng/ml, the second consisted of 21 samples with I-50 ranging from 1.51 t o 3 ng/ml, and the third of 17 samples with I-50 ranging from 3.01 to 13 ng /ml. The determination coefficient (R-2 = 0.999) showed that the data obtai ned by gas chromatography represented a reliable basis. The results obtaine d by ELISA were also reliable, with an estimated Pearson correlation coeffi cient of 0.82 between the two methods. This coefficient for the different g roups (0.88, 0.79 and 0.49 for groups 1, 2 and 3, respectively) showed grea ter reliability for the test with dilutions closer to I-50.