The use of adjustment factors to address the impact of inflammation on vitamin A and iron status in humans.

Many nutrient biomarkers are altered by inflammation. We calculated adjustment factors for retinol and ferritin by using meta-analyses of studies containing the respective biomarker and 2 acute phase proteins in serum, C-reactive protein (CRP), and α1-acid glycoprotein (AGP). With the use of CRP and AGP we identified 4 groups in each study: reference (CRP ≤5 mg/L, AGP ≤1 g/L), incubation (CRP >5 mg/L, AGP ≤1 g/L), early convalescence (CRP >5 mg/L, AGP >1 g/L), and late convalescence (CRP ≤5 mg/L, AGP >1 g/L). For each biomarker, ratios of the geometric means of the reference to each inflammation group concentration were used to calculate adjustment factors for retinol (1.13, 1.24, and 1.11) and ferritin (0.77, 0.53, and 0.75) for the incubation, early, and late convalescent groups, respectively. The application of the meta-analysis factors in more recent studies compares well with study-specific factors. The same method was used to calculate adjustment factors for soluble transferrin receptor (sTfR) and body iron stores (BISs) in Lao children. We found no advantage in adjusting sTfR for inflammation; in fact, adjustment decreased iron deficiency. Neither adjusted (10% <0 mg/kg) nor nonadjusted (12% <0 mg/kg) BISs detected as much iron deficiency as did ferritin (18% <12 μg/L) and adjusted ferritin (21% <12 μg/L) unless the cutoff for BISs was increased from 0 to <3 mg/kg. However, we could find no evidence that the larger number of children identified as having BISs <3 mg/kg had risks of anemia comparable to those identified by using ferritin <12 μg/L. In conclusion, both corrected and uncorrected ferritin concentrations <12 μg/L are associated with more iron deficiency and anemia than either sTfR >8.3 mg/L or BISs <0 mg/kg in Lao children.