Abstract: Disclosed herein are methods of diagnosing, monitoring and treating iron disorders using assays of serum ferritin. Some embodiments relate to detection and management of iron and iron-containing proteins in the body. For instance, the level of ferritin may be determined by contacting a sample with one or more magnetic nanoparticles; measuring an absorbance of the sample in the presence and in the absence of a magnetic field; and comparing the absorbance of the sample in the presence of the magnetic field to the absorbance of the sample in the absence of the magnetic field to determine the level of ferritin in the sample.