Biocompatible ferrofluids have demonstrated considerable potential in biomedical applications, particularly in magnetic drug targeting, owing to their magnetically controllable rheological properties. However, the evaluation of magnetoviscous effects (MVEs) in complex biological environments, such as blood, remains challenging, especially when baseline viscosities differ significantly between systems. In this study, the MVE of a starch-coated ferrofluid (FluidMAG-D200) diluted in water, sheep blood, pig blood, and sheep plasma was systematically investigated at low shear rates relevant to microvascular flow. To evaluate the magnetoviscous behavior more clearly, we applied a dual-metric evaluation strategy, which combines conventional analysis of relative viscosity change (Δ η _R) with a novel approach based on absolute stress increments. The results reveal that the analysis of Δ η _R is strongly influenced by the baseline viscosity of the mixtures and may therefore misrepresent the actual magnitude of magnetically induced structuring. In contrast, absolute stress increments analysis provides a more consistent basis for comparison across systems with differing viscosities. Notably, while water-based mixtures exhibit the highest relative changes, blood-based systems—particularly pig blood mixtures—show larger absolute viscosity increases under magnetic fields. These findings highlight the importance of evaluation methodology in interpreting MVE in complex magnetic fluids. Additionally, shear-dependent differences between blood-based systems suggest a possible influence of red blood cells on field-induced microstructural formation, although direct interactions cannot be conclusively determined within the scope of this study.