Download Appendix 3. Calculation of free and bioavailable 25(OH)D (30, 39

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Appendix 3. Calculation of free and bioavailable 25(OH)D (30, 39)
[Dtotal] = serum 25(OH)D concentration
[Alb] = serum albumin concentration
[DBP] = serum vitamin D binding protein concentration
[DAlb] = albumin-bound 25(OH)D concentration
[DDBP] = DBP-bound 25(OH)D concentration
[Dfree] = free 25(OH)D concentration
[Dbioavailable] = bioavailable 25(OH)D concentration = [Dfree] + [DAlb]
Kalb = affinity constant between vitamin D and albumin = 6 x 105 M-1 for 25-OH D
KDBP = affinity constant between vitamin D and DBP = 7 x 108 M-1 for 25-OH D
Equation
1
[DDBP] =
[Dtotal] – [DAlb] – [Dfree]
2
[DAlb] =
Kalb  [Alb]  [Dfree]
3
[Dfree] =
[DDBP]  KDBP  [DBPfree]
4
[DBPfree] =
[DBPtotal] – [DDBP]
From equations 3 and 4
[Dfree] =
[DDBP]  KDBP  ([DBP] – [DDBP])
From equations 1 and 2
[DDBP] =
[Dtotal] – (Kalb  [Alb] +1)  [Dfree]

[Dfree] =


Simplified to fit a second-degree polynomial (ax2 + bx + c = 0) where x=[Dfree]:
a=
KDBP  Kalb  [Alb] + KDBP
b=
KDBP  [DBP] – KDBP  [Dtotal] + Kalb  [Alb] +1
c=
-[Dtotal]
[Dfree] =
[Dbioavailable] =
=
[-b + √
]  2a
[Dfree] + [DAlb]
(Kalb  [Alb] + 1)  [Dfree]

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