Particle counting immunoassay (PACIA) of ferritin

Particle Counting ImmunoAssay (PACIA) has been adapted to the determination of serum ferritin. Polystyrene particles (0.8 micrometers) were coated with anti-ferritin antibodies and the concentration of ferritin determined by the agglutinating activity of this protein. The agglutination was measured by the reduction of the number of non-agglutinated particles counted in an optical blood cell counter. Non-specific agglutination and inhibition of agglutination were prevented in 99% of clinical samples by the use of F(ab')2 fragments of the antiserum IgG to coat the particles and by using slightly dissociating conditions (pH 9.2, ammonium thiocyanate, EDTA). The system was automated with a sampling rate of 50/h and an incubation time of 25 min. The standard curve ranged from 1 to 100 micrograms/l; recoveries were between 93.6 to 100.2%; the correlation coefficients of PACIA with radioimmunoassays calculated respectively on 99 and 91 samples were 0.974 and 0.984; maximal within- and between assay CV were 11.2% and 7.7%.