Table 1.

Summary of key figures of merit for the determination of 25OH-D in plasma and serum extracts by DI-MS/MS with 2-NO-Pyr click derivatization

Figures of merit	25OH-D3	25OH-D2
Mean recovery (range)a	103% (92–112%)	101% (93–113%)
Precision (intraday; interday)b	10.9%; 10.5%	N/A; N/A
Linearity (slope; R2)c	0.0037; 0.992	0.0037; 0.996
LOQ (S/N = 10)	3.9 nmol/l (1.7 pg)	3.6 nmol/l (1.6 pg)
LOD (S/N = 3)	1.2 nmol/l (0.52 pg)	1.1 nmol/l (0.50 pg)
% Detects in reference blood samples (n = 18)d	100%	21%
% Detects in pediatric plasma samples (n = 30)e	100%	43%

LOD, limit of detection; LOQ, limit of quantification; N/A, not available.

^aSpike-and-recovery experiments performed at three concentration levels (15, 30, and 45 nmol/l) in triplicate, showing the average and range of 25OH-D recoveries.

^bIntraday precision, summarized as coefficient of variation, was determined by repeatedly analyzing a processed plasma sample (n = 412) over 24 h. Interday precision was determined by sequentially analyzing a processed plasma sample 50 times a day for 3 days. The blood samples used for these experiments had 25OH-D2 below the LOQ of the DI-MS/MS method.

^cExternal calibration curves were normalized to d6-25OH-D3 over a 12.5-fold calibration range using six calibrants where slope units are in (nmol/l)⁻¹.

^dDetection frequency >LOD in reference materials from NIST and DEQAS (n = 18).

^eDetection frequency >LOD in VITdALIZE-KIDS plasma samples (n = 30).