Secretory IgA enzyme immunoassay. Application of a model for computation of the standard curve.

An enzyme-linked immunosorbent assay (ELISA) is applied for quantitation of secretory IgA (SIgA) in nasopharyngeal secretions (NPS) and saliva. In principle, both the alpha and secretory component determinants of SIgA are utilized in a two-site sandwich technique. Dilutions of a SIgA solution prepared from colostrum (Col-SIgA) are used for standards. A continuous SIgA standard curve, describing the relationship between the absorbance of the coloured end product and the corresponding Col-SIgA dilution, is computed by means of a fitting procedure based on a model. The variation (SD) of the absorbance was found to be proportional to the absorbance level. Knowing the SD of the absorbance, the total variation of the SIgA estimates could be calculated experimentally by means of Monte Carlo methods. Based on these results, the analytical range was chosen from 2000 micrograms/l to 50 micrograms/l of SIgA. Inhibition experiments revealed that plasma IgA could be added to the NPS in equal amounts to the SIgA content without it interfering with the SIgA measurements. Percentage recovery (median) of known amounts of SIgA added to NPS was 96%. In NPS from children suffering from recurrent upper respiratory tract infections the median SIgA level was 1.44 g/l (interquartile range 0.61-2.43 g/l); in saliva from healthy children the median SIgA content was 77 mg/l (interquartile range 56-182 mg/l).