Table 5.
Central 95% reference intervals for serum folate forms and for serum and red blood cell total folate in the US population ≥1 year, NHANES 2011–2a
| All persons | Fasted persons | Nonfasted persons | ||||
|---|---|---|---|---|---|---|
| Analyte | 2.5th | 97.5th | 2.5th | 97.5th | 2.5th | 97.5th |
| (nmol/L) | ||||||
| Serum 5-methylTHF | 12.3 (11.4–13.2) | 95.4 (92.0–99.8) | 11.7 (10.8–12.5) | 96.1 (91.4–104) | 13.2 (11.6–13.9) | 94 (91.5–96.6) |
| Serum UMFA | .410 (.368–.449) | 17.2 (14.3–21.0) | .383 (.341–.416) | 4.93 (2.99–9.70) | .450 (.397–.480) | 28.0 (219–37.7) |
| Serum non-methyl folate | <LOD | 5.94 (5.31–7.53) | <LOD | 5.89 (5.06–7.69) | <LOD | 6.08 (5.50–7.62) |
| THF | <LOD | 5.38 (4.74–7.16) | <LOD | 5.37 (4.55–7.37) | <LOD | 5.39 (4.83–7.34) |
| 5-FormylTHF | <LOD | .340 (<LOD–.392) | <LOD | .336 (<LOD–.406) | <LOD | .343 (.317–.383) |
| 5,10-MethenylTHF | <LOD | .478 (.414–.570) | <LOD | .480 (–) | <LOD | .629 (.532–.775) |
| Serum MeFox | .445 (.401–.475) | 6.13 (5.70–6.77) | .384 (<LOD–.424) | 4.75 (4.20–5.55) | .536 (.496–.565) | 7.01 (6.66–7.59) |
| Serum total folate | ||||||
| Excluding MeFox | 14.7 (13.6–15.9) | 107 (103–112) | 14.1 (13.3–14.8) | 103 (98.6–111) | 16.0 (14.5–17.0) | 111 (104–124) |
| Including MeFox | 16.0 (14.5–16.9) | 109 (106–115) | 14.7 (13.8–16.0) | 105 (100–113) | 17.1 (16.4–18.0) | 114 (108–126) |
| RBC total folate | 477 (460–493) | 2320 (2200–2520) | 476 (445–505) | 2340 (2250–2480) | 477 (460–492) | 2300 (2100–2560) |
NHANES, National Health and Nutrition Examination Survey; 5-methylTHF, 5-methyltetrahydrofolate; UMFA, unmetabolized folic acid; THF, tetrahydrofolate; 5-formylTHF, 5-formyltetrahydrofolate; 5,10-methenylTHF, 5,10-methenyltetrahydrofolate; MeFox, pyrazino-s-triazine derivative of 4α-hydroxy-5-methylTHF; RBC, red blood cell
Values are percentiles (95% CI); serum folate forms and serum total folate (sum of folate forms excluding or including MeFox) were measured by HPLC-MS/MS; RBC total folate was measured by microbiologic assay; non-methyl folate represents sum of three minor forms: THF, 5-formylTHF, and 5,10-methenylTHF; for sample sizes, see Table 1.