Table 3.

Observational and genetic effect estimates for CRP (mg/L) and complement C3 (g/L) by ferritin concentration (μg/L).

Observational estimatesaGenetic effect estimatesb
Adjusted logistic regressionExtreme genotype score
NOdds ratio95% CINOdds ratio95% CI
CRP ≥2 vs <2 mg/L
Complement C3 >1.04 vs ≤1.04 g/L
  • Individuals with CRP concentration ≥10 mg/L were excluded (N = 2633) to ensure that none of the participants had an ongoing infection or severe inflammation because of chronic disease. In the analysis concerning ferritin concentration, individuals with either ferritin concentration <20 μg/L or iron concentration <60 μg/dL (<10 μmol/L) were excluded (N = 8448) to ensure that none of the individuals had iron deficiency.

  • a Observational estimates are obtained from logistic regression adjusted for sex (not in sex-stratified analyses), age (<50 vs ≥50 years), alcohol intake (consumption of ≤7 units/week vs >7 units/week), BMI (<25 vs ≥25 kg/m2), smoking (current vs nonsmoker), diabetes (yes vs no), and menopausal status (yes vs no in stratified analyses with women). Ferritin was on continues scale and was logarithmically transformed [log2 (ferritin)] because of skewness of the distribution.

  • b We performed an instrumental variable analysis by constructing an extreme genotype score comparing the high-risk genotype C282Y/C282Y with wild type/wild type (reference group) using a ratio estimate to calculate the log odds ratio of increased CRP or complement C3 per unit increase in log2 (ferritin) concentration. We obtained 95% CI using Fieller's method.