. 2016 Mar 9;7(2):349–356. doi: 10.3945/an.115.010215

TABLE 3.

Proposed methods to interpret ferritin values in the presence of inflammation¹

Proposed methods
Higher ferritin cutoff: Use a ferritin cutoff of <30 μg/L instead of <12 μg/L among populations with inflammation to define deficiency (18).
Exclude individuals: Stratify participants into those without and with inflammation (CRP > 5 mg/L and/or AGP > 1 g/L) and use ferritin values only among those without inflammation (19).
ICFs: Stratify individuals into those with and without inflammation. In the 4-group model, individuals are defined by no inflammation (CRP < 5 mg/L and AGP < 1 g/L), the incubation period (CRP > 5 mg/L), early convalescence (CRP > 5 mg/L and AGP > 1 g/L), and late convalescence (AGP > 1 g/L). A 2-group model can be used when only CRP or AGP was measured. ICFs are then generated by dividing the ferritin GM values of each inflammation group by the mean values in the non-inflammation group (26): ICF_i = GM_ref/GM_i. Subsequently, the ferritin values in individuals in the raised inflammation groups are multiplied by the ICFs matching their respective inflammation group.
Meta-analysis correction factors: Use the same approach as the ICF method but with the use of external correction factors generated from a meta-analysis (26).
Regression modeling: Run a linear regression analysis with ferritin as the dependent variable and CRP and/or AGP as the independent variables and use the slope of CRP and/or AGP to adjust for the effect of inflammation. A CRP and AGP reference value (e.g., maximum of lowest decile) may be required so that ferritin is not overadjusted at the lower values for CRP and AGP. Adjustment would be performed by using the following equation: Adjusted ferritin = unadjusted ferritin − β₁(CRP_obs − CRP_ref) − β₂(AGP_obs − AGP_ref).

AGP, α₁-acid glycoprotein; CRP, C-reactive protein; GM, geometric mean; GM_i = geometric mean of those in the incubation, early, and late convalescence groups; GM_ref = geometric mean of those without inflammation; ICF, internal correction factor; obs, observed; ref, reference.