Immunomagnetic reduction (IMR), which involves measuring the reduction in the ac magnetic susceptibility of magnetic reagents, is due to the association between bio-functionalized magnetic nanoparticles and target bio-molecules. This has been demonstrated for assaying proteins in solutions free of Fe ions, such as serum. In this work, the validity of IMR assay for samples rich in Fe ions like hemoglobin (Hb) is investigated. According to the results, there is nomagnetic signal contributed by Fe-ion-rich Hb. Furthermore, the results show a high sensitivity in assaying hemoglobin A1c (HbA1c) by using IMR.