CRP and hydrops fetalis: The combined use of serum ferritin with inflammatory markers such as erythrocyte sedimentation rate (ESR) or C-reactive protein (CRP) in a discriminant analysis provide only marginal improvement in sensitivity/specificity.42 Serum ferritin < 100 μg/l has been widely used as a cut-off in high-income countries when looking for absolute ID in patients with HF in most clinical trials.