"Sandwich" enzyme immunoassay for serum ferritin with polypropylene test tubes as the solid phase

A "sandwich" enzyme immunoassay for ferritin in human serum is described, in which horseradish peroxidase-labeled antibody and a very sensitive chromogen, 2,2'-azino-di(3-ethyl-benzthiazolin-6-sulfonate), are used. The solid phase is polypropylene test tubes treated with glutaraldehyde; immunologic reactions are developed during constant rotation of the tubes. The assay requires 20 microL of serum per assay and takes less than 3 h. The standard curve is linear between 0 and 400 micrograms of serum ferritin per liter. Analytical recoveries of various amounts of standard added to a serum sample ranged from 92 to 98.6%. The sensitivity is 2 micrograms/L, which translates to 0.4 ng per assay tube. Within-run CV ranges from 1.6 to 5.4% and between-run from 4.2 to 7.8% according to the ferritin concentration. Results by competitive radioimmunoassay correlated well (r = 0.98). No "hook effect" was observed for concentrations up to 13 mg/L. The reference interval is 40-240 micrograms/L for men, 8-100 micrograms/L for women.