Development of an automated immunoturbidimetric ferritin assay

We have developed a new procedure for turbidimetric measurement of ferritin concentration in human serum, based on latex microparticle agglutination technology. The procedure has been automated using the Falcor 300 analyzer. Carboxilated latex particles (336 nm in diameter) were covalently coupled with immunopurified F(ab')2 fragments of anti-ferritin IgG antibodies. Coated microparticles were automatically mixed with undiluted sample and the resulting absorbance due to agglutination was measured at 550 nm. The procedure generated a calibration curve with a measuring range of 0 to 558 microg/l, showing a day-to-day imprecision lower than 5.7%. The detection limit was 4 microg/l. There were no interferences from bilirubin, hemoglobin or rheumatoid factors. Turbid and lipemic samples caused an important interference which could be avoided by pretreating those samples prior to measurement. A prozone effect was provisionally obtained with ferritin concentrations over 1800 microg/l. The results suggested a hook-like effect due to a rapid microparticle precipitation in the reaction media, that could be avoided by increasing the reaction medium density by adding sucrose to the buffer, up to 150 g/l concentration. This sucrose addition resulted in a displacement of the Heidelberger curve with a prozone phenomenon occuring at concentration higher than 3000 microg/l of ferritin. Results obtained with the present procedure correlated well with those obtained by a nephelometric procedure and with those obtained by an RIA. We conclude that this latex turbidimetric immunochemical procedure is simple, rapid, has a good analytical and operational performance on the Falcor 300 analyzer and is well suited for the measurement of ferritin concentration in human serum.