SF (μg/L) concentration thresholds (95% confidence interval in parentheses) identified by Hb (g/dL) and sTfR1 (mg/L) using RCS regression with 5 knots in an apparently healthy sample of US pregnant women aged 15 to 49 years participating in the NHANES 1999 to 2010 and 2015 to 2018
| . | Total (N = 1288) . | First trimester (n = 207) . | Second trimester (n = 436) . | Third trimester (n = 409) . | Unknown trimester (n = 248) . |
|---|---|---|---|---|---|
| Hb | |||||
| SF corresponding to median Hb plateau point | 19.5 (18.1-21.6) | 25.8 (18.1-28.5)† | 18.3 (16.3-22.9)‡ | 19.0 (14.4-26.1)‡ | 18.6 (16.4-25.6)‡ |
| RCS model adjusted R, % | 12.8 | 7.6 | 12.8 | 9.9 | 13.9 |
| sTfR1 | |||||
| SF corresponding to median sTfR1 minima point | 18.9 (18.2-19.7) | 23.2 (19.7-28.0)† | 19.9 (18.2-23.5)‡ | 20.5 (15.5-21.9)‡ | 17.8 (16.6-19.6)‡ |
| RCS model adjusted R, % | 27.3 | 23.5 | 33.1 | 22.7 | 21.5 |
| . | Total (N = 1288) . | First trimester (n = 207) . | Second trimester (n = 436) . | Third trimester (n = 409) . | Unknown trimester (n = 248) . |
|---|---|---|---|---|---|
| Hb | |||||
| SF corresponding to median Hb plateau point | 19.5 (18.1-21.6) | 25.8 (18.1-28.5)† | 18.3 (16.3-22.9)‡ | 19.0 (14.4-26.1)‡ | 18.6 (16.4-25.6)‡ |
| RCS model adjusted R, % | 12.8 | 7.6 | 12.8 | 9.9 | 13.9 |
| sTfR1 | |||||
| SF corresponding to median sTfR1 minima point | 18.9 (18.2-19.7) | 23.2 (19.7-28.0)† | 19.9 (18.2-23.5)‡ | 20.5 (15.5-21.9)‡ | 17.8 (16.6-19.6)‡ |
| RCS model adjusted R, % | 27.3 | 23.5 | 33.1 | 22.7 | 21.5 |
Unweighted n and analyses. The following exclusions apply to define a healthy sample: pregnant women with inflammation (C-reactive protein, >5.0 mg/L) or possible liver disease (alanine aminotransferase, >70 U/L or aspartate aminotransferase, >70 U/L). All plateau and minima estimates, along with their respective 95% CIs, were obtained from 5000 bootstrap replicates. All CIs have been corrected for bias using the bias corrected acceleration approach.39 Column values with different symbols (†, or ‡) are significantly different, P < .05.