Quantification of human urinary free secretory component, secretory and nonsecretory IgA by ELISA

The specific quantification of human urinary free secretory component (FSC), secretory IgA (SIgA) and total IgA using ELISA has been hampered by mutual interferences of these three molecules. Using affinity chromatographically purified antisera an attempt was therefore made to reduce these interferences without necessitating further assay steps. FSC and total IgA were measured in unprocessed urine by means of anti-FSC and anti-IgA as well as alkaline phosphatase-coupled anti-FSC or anti-IgA antisera. SIgA was determined using anti-IgA as well as alkaline phosphatase-coupled anti-FSC. Nonsecretory urinary IgA was calculated from the measured SIgA and total IgA. The mutual interferences of FSC, SIgA or nonsecretory IgA in the three assay systems were low and not relevant for normal samples. Normal urinary concentrations were: FSC 344 +/- (SD) 208 ng/ml (n = 120), SIgA 1,874 +/- 1,133 ng/ml (n = 123) and nonsecretory IgA, depending on the way of standardization, 712 +/- 699 (n = 56) or 878 +/- 732 ng/ml (n = 51). SIgA excretion increased with age. Lower urinary SIgA as well as total and nonsecretory IgA levels were observed in males as compared to females. No correlation evolved between the hormonal status of women and the excretion of FSC, SIgA or IgA. In IgA-deficient patients virtually no nonsecretory IgA or SIgA was detected in the urine while the FSC concentration was in the normal range.