The present disclosure introduces methods for characterizing iron core carbohydrate colloid drug products, such as iron sucrose drug products. Disclosed methods enable the characterization of the iron core size of the iron core nanoparticles in iron carbohydrates as they exist in the formulation in solution, such as e.g. iron sucrose drug products, and more particularly, the average particle diameter size and size distribution(s) of the iron core nanoparticles. The disclosed methods apply small-angle X-ray scattering (SAXS) in parallel beam transmission geometry, with a sample mounted inside a capillary and centered in the X-ray beam, to iron carbohydrates, such as iron sucrose, in solution without the need to modify the sample, such as to remove unbound carbohydrates, dilute, or dry the sample, to accurately characterize the average iron core particle diameter size of the iron core nanoparticles. An example application of the disclosed method is to perform SAXS measurements under identical instrument settings on two samples of the same type of iron core nanoparticle colloid drug product for the purpose of comparing their iron core structures. Such comparisons are typically performed during the iron core carbohydrate colloid drug development process, and can include comparisons of samples that have been manipulated.