Participant characteristics and serum ferritin concentration thresholds identified by restricted cubic spline regression in a healthy sample of US nonpregnant women aged 20 to 49 years from REDS-RISE and NHANES data
| Participant characteristics . | REDS-RISE (N = 286) . | NHANES (N = 5442) . | P . |
|---|---|---|---|
| Age, mean (SD), y | 33.8 (8.9) | 34.6 (8.6) | .55 |
| Hemoglobin (g/dL), mean (SD) | 13.2 (1.0) | 13.2 (1.3) | .45 |
| Serum ferritin (µg/L), median (25th, 75th) | 33.0 (21.0, 52.0) | 34.4 (18.0, 59.0) | .73 |
| Soluble transferrin receptor (mg/L), median (25th, 75th) | 2.7 (2.2, 3.1) | 3.2 (2.6, 4.1) | <.01 |
| Serum ferritin (SF, µg/L) concentration thresholds | |||
| Soluble transferrin receptor (sTfR) | |||
| SF (µg/L) corresponding to sTfR minimum point | 25.4 (23.1, 30.0)* | 25.4 (24.7, 26.3)† | .98 |
| RCS model-adjusted R2, % | 13.0 | 40.3 | |
| Hemoglobin | |||
| SF (µg/L) corresponding to Hb plateau point | 25.3 (13.2, 37.4)‡ | 26.6 (25.6, 27.9)§ | .83 |
| RCS model-adjusted R2, % | 8.0 | 35.4 | |
| Participant characteristics . | REDS-RISE (N = 286) . | NHANES (N = 5442) . | P . |
|---|---|---|---|
| Age, mean (SD), y | 33.8 (8.9) | 34.6 (8.6) | .55 |
| Hemoglobin (g/dL), mean (SD) | 13.2 (1.0) | 13.2 (1.3) | .45 |
| Serum ferritin (µg/L), median (25th, 75th) | 33.0 (21.0, 52.0) | 34.4 (18.0, 59.0) | .73 |
| Soluble transferrin receptor (mg/L), median (25th, 75th) | 2.7 (2.2, 3.1) | 3.2 (2.6, 4.1) | <.01 |
| Serum ferritin (SF, µg/L) concentration thresholds | |||
| Soluble transferrin receptor (sTfR) | |||
| SF (µg/L) corresponding to sTfR minimum point | 25.4 (23.1, 30.0)* | 25.4 (24.7, 26.3)† | .98 |
| RCS model-adjusted R2, % | 13.0 | 40.3 | |
| Hemoglobin | |||
| SF (µg/L) corresponding to Hb plateau point | 25.3 (13.2, 37.4)‡ | 26.6 (25.6, 27.9)§ | .83 |
| RCS model-adjusted R2, % | 8.0 | 35.4 | |
The following inclusion criteria were used to identify apparently healthy populations. For REDS-RISE,6 all women were considered healthy based on being eligible volunteer blood donors who underwent clinical screening and were accepted for blood donation. For NHANES, as previously described, nonpregnant women were without inflammation, infection (WBC > 109), or possible liver disease (AST/ALT).2 All plateau and minimum estimates and their 95% confidence interval (CI) were obtained from 5000 bootstrap replicates. All CIs have been corrected for bias using the bias corrected acceleration (BCa) approach.25 Column estimates with different superscripts indicate pairwise values that are statistically different from each other at P < .05 (*vs ‡, within REDS-RISE P difference = .99; † vs §, within NHANES P difference = .10). P values calculated from t test for age and Hb and Wilcoxon median tests for SF and sTfR.