Table 6.
Effect of long-term storage at suboptimal freezing temperature (−20°C) for up to 12 mo on serum folate concentrations1
| Analyte QC pool | Serum folate concentration2 | Relative change3 | ||||
|---|---|---|---|---|---|---|
| Reference | 3 mo | 6 mo | 9 mo | 12 mo | 3 mo | |
| nmol/L | % | |||||
| tFOL | 4.91* (−7.92, −1.81) | |||||
| Low | 17.5 | 16.8 | 15.9 | 15.5 | 15.3 | |
| Medium | 40.2 | 38.2 | 36.6 | 36.3 | 35.1 | |
| High | 63.7 | 59.9 | 58.5 | 57.5 | 56.2 | |
| 5-MethylTHF | −9.58** (−12.9, −6.13) | |||||
| Low | 14.8 | 13.7 | 12.4 | 12.0 | 11.8 | |
| Medium | 32.6 | 28.9 | 27.0 | 26.6 | 25.5 | |
| High | 48.2 | 43.5 | 40.3 | 39.2 | 38.2 | |
| Folic acid | −2.13 (−7.88, 3.99) | |||||
| Low | 0.84 | 0.80 | 0.83 | 0.80 | 0.84 | |
| Medium | 3.88 | 3.96 | 3.86 | 3.96 | 3.92 | |
| High | 9.16 | 8.80 | 8.99 | 9.12 | 9.00 | |
| MeFox | ||||||
| Low | 0.62 | 1.03 | 1.44 | 1.49 | 1.42 | 66.9** (34.4, 107) |
| Medium | 0.86 | 2.45 | 2.90 | 2.80 | 2.87 | 187** (131, 256) |
| High | 1.90 | 3.09 | 4.70 | 4.57 | 4.44 | 62.7** (31.0, 102) |
1
Serum QC pools stored at −70°C were used as reference; statistical analysis by 2-way ANOVA. 5-methylTHF, 5-methyltetrahydrofolate; MeFox, pyrazino-s-triazine derivative of 4α-hydroxy-5-methyltetrahydrofolate; QC, quality control; tFOL, total folate.
2
Values are geometric means from 6 replicates/condition
3
Values are percent change (95% Cl) compared to reference across the QC pools except for MeFox where a significant interaction between time and pool (P-interaction = 0.0299) necessitated providing separate estimates for each pool
*
0.0001 ≥ P < 0.05;
**
P <0.0001