Table 4.
Effect of repeated freeze/thawing of closed vials on serum folate concentrations1
| Analyte QC pool | Serum folate concentration2 | Relative change3 | |||||
|---|---|---|---|---|---|---|---|
| Reference | 1 F/T | 2 F/T | 3 F/T | 1 F/T | 2 F/T | 3 F/T | |
| nmol/L | % | ||||||
| tFOL | 1.17 (−0.59, 2.97) | −0.95 (−2.67, 0.82) | 0.83 (−0.93, 2.62) | ||||
| Low | 16.4 | 16.7 | 16.3 | 16.5 | |||
| Medium | 37.3 | 37.3 | 36.5 | 37.0 | |||
| High | 57.9 | 58.8 | 57.9 | 59.3 | |||
| 5-MethylTHF | 0.86 (−0.85, 2.60) | −1.62 (−3.29, 0.08) | −0.92 (−2.60, 0.79) | ||||
| Low | 14.6 | 14.8 | 14.5 | 14.6 | |||
| Medium | 32.6 | 32.3 | 31.7 | 31.8 | |||
| High | 46.9 | 47.9 | 46.6 | 47.0 | |||
| Folic acid | 1.20 (−3.09, 5.68) | −1.31 (−5.50, 3.06) | 5.24* (0.78, 9.90) | ||||
| Low | 0.81 | 0.84 | 0.81 | 0.87 | |||
| Medium | 3.37 | 3.52 | 3.30 | 3.48 | |||
| High | 7.96 | 7.61 | 7.78 | 8.36 | |||
| MeFox | 10.2* (1.58, 19.5) | 14.6* (5.62, 24.3) | 26.0** (16.2, 36.7) | ||||
| Low | 0.96 | 1.05 | 0.99 | 1.08 | |||
| Medium | 1.25 | 1.41 | 1.53 | 1.67 | |||
| High | 2.95 | 3.19 | 3.51 | 3.91 | |||
1
2 h per freeze/thawing cycle at 22°C; serum QC pools stored at −70°C were used as reference; statistical analysis by 2-way ANOVA. 5-methylTHF, 5-methyltetrahydrofolate; F/T, freeze/thawing; MeFox, pyrazino-s-triazine derivative of 4α-hydroxy-5-methyltetrahydrofolate; QC, quality control; tFOL, total folate.
2
Values are geometric means from 4 replicates/condition
3
Values are percent change (95% Cl) compared to reference across the QC pools (no significant interaction between time and pool)
*
0.0001 ≥ P < 0.05;
**
P <0.0001