TABLE 7.
| Vial | 25(OH)D3 | 25(OH)D2 | C-3 epimer of 25(OH)D3 |
| nmol/L | |||
| Level 1: Normal human serum | 59.6 ± 2.1 | – | – |
| Level 2: 1 diluted 1:1 with horse serum | 30.8 ± 1.5 | 4.14 ± 0.19 | – |
| Level 3: Normal human serum + 25(OH)D2 | 46.2 ± 2.8 | 64.1 ± 4.8 | – |
| Level 4: 1 + 25(OH)D2 + C-3 epimer of 25(OH)D3 | 82.3 ± 2.0 | 5.81 ± 0.52 | 94.1 ± 2.9 |
Values are certified concentration values plus or minus the expanded uncertainty. Expanded uncertainty is the uncertainty in the certified value, calculated according to the measurement procedure described in the International Organization for Standardization Guide, is expressed as an expanded uncertainty, . The expanded uncertainty is calculated as U = kuc, where uc is intended to represent, at the level of 1 SD, the combined effect of between-laboratory and within-laboratory components of uncertainty. The coverage factor (k) is determined from the Student's t-distribution corresponding to the appropriate associated degrees of freedom, and ∼95% confidence for each analyte (15).
Complete information on the development, use, and interpretation of the SRM 972 materials can be found in the NIST Certificate of Analysis (15).
For 25(OH)D concentration values, molar concentration levels were calculated from mass concentration levels using the relative molecular masses. The equivalent factors for converting ng/mL to nmol/L are 2.4233 for 25(OH)D2 and 2.4959 for 25(OH)D3 and 3-epi-25(OH)D3 (15).