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result sql = Measurement of 25-OH-vitamin D in human serum using liquid chromatography tandem-mass spectrometry with comparison to radioimmunoassay and automated immunoassay.
Measurement of 25-OH-vitamin D in human serum using liquid chromatography tandem-mass spectrometry with comparison to radioimmunoassay and automated immunoassay.
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char = h;code = 104
char = i;code = 105
char = c;code = 99
char = ;code = 32
char = s;code = 115
char = e;code = 101
char = p;code = 112
char = a;code = 97
char = r;code = 114
char = a;code = 97
char = t;code = 116
char = i;code = 105
char = o;code = 111
char = n;code = 110
char = ;code = 32
char = o;code = 111
char = n;code = 110
char = ;code = 32
char = a;code = 97
char = ;code = 32
char = C;code = 67
char = -;code = 45
char = 1;code = 49
char = 8;code = 56
char = ;code = 32
char = c;code = 99
char = o;code = 111
char = l;code = 108
char = u;code = 117
char = m;code = 109
char = n;code = 110
char = ;code = 32
char = u;code = 117
char = s;code = 115
char = i;code = 105
char = n;code = 110
char = g;code = 103
char = ;code = 32
char = a;code = 97
char = ;code = 32
char = w;code = 119
char = a;code = 97
char = t;code = 116
char = e;code = 101
char = r;code = 114
char = /;code = 47
char = m;code = 109
char = e;code = 101
char = t;code = 116
char = h;code = 104
char = a;code = 97
char = n;code = 110
char = o;code = 111
char = l;code = 108
char = /;code = 47
char = a;code = 97
char = m;code = 109
char = m;code = 109
char = o;code = 111
char = n;code = 110
char = i;code = 105
char = u;code = 117
char = m;code = 109
char = ;code = 32
char = a;code = 97
char = c;code = 99
char = e;code = 101
char = t;code = 116
char = a;code = 97
char = t;code = 116
char = e;code = 101
char = ;code = 32
char = g;code = 103
char = r;code = 114
char = a;code = 97
char = d;code = 100
char = i;code = 105
char = e;code = 101
char = n;code = 110
char = t;code = 116
char = .;code = 46
char = ;code = 32
char = D;code = 68
char = e;code = 101
char = t;code = 116
char = e;code = 101
char = c;code = 99
char = t;code = 116
char = i;code = 105
char = o;code = 111
char = n;code = 110
char = ;code = 32
char = w;code = 119
char = a;code = 97
char = s;code = 115
char = ;code = 32
char = b;code = 98
char = y;code = 121
char = ;code = 32
char = A;code = 65
char = t;code = 116
char = m;code = 109
char = o;code = 111
char = s;code = 115
char = p;code = 112
char = h;code = 104
char = e;code = 101
char = r;code = 114
char = i;code = 105
char = c;code = 99
char = ;code = 32
char = P;code = 80
char = r;code = 114
char = e;code = 101
char = s;code = 115
char = s;code = 115
char = u;code = 117
char = r;code = 114
char = e;code = 101
char = ;code = 32
char = E;code = 69
char = l;code = 108
char = e;code = 101
char = c;code = 99
char = t;code = 116
char = r;code = 114
char = o;code = 111
char = s;code = 115
char = p;code = 112
char = r;code = 114
char = a;code = 97
char = y;code = 121
char = ;code = 32
char = I;code = 73
char = o;code = 111
char = n;code = 110
char = i;code = 105
char = s;code = 115
char = a;code = 97
char = t;code = 116
char = i;code = 105
char = o;code = 111
char = n;code = 110
char = ;code = 32
char = (;code = 40
char = A;code = 65
char = P;code = 80
char = -;code = 45
char = E;code = 69
char = I;code = 73
char = );code = 41
char = ;code = 32
char = f;code = 102
char = o;code = 111
char = l;code = 108
char = l;code = 108
char = o;code = 111
char = w;code = 119
char = e;code = 101
char = d;code = 100
char = ;code = 32
char = b;code = 98
char = y;code = 121
char = ;code = 32
char = s;code = 115
char = e;code = 101
char = l;code = 108
char = e;code = 101
char = c;code = 99
char = t;code = 116
char = e;code = 101
char = d;code = 100
char = ;code = 32
char = r;code = 114
char = e;code = 101
char = a;code = 97
char = c;code = 99
char = t;code = 116
char = i;code = 105
char = o;code = 111
char = n;code = 110
char = ;code = 32
char = m;code = 109
char = o;code = 111
char = n;code = 110
char = i;code = 105
char = t;code = 116
char = o;code = 111
char = r;code = 114
char = i;code = 105
char = n;code = 110
char = g;code = 103
char = .;code = 46
char = ;code = 32
char = W;code = 87
char = e;code = 101
char = ;code = 32
char = c;code = 99
char = o;code = 111
char = m;code = 109
char = p;code = 112
char = a;code = 97
char = r;code = 114
char = e;code = 101
char = d;code = 100
char = ;code = 32
char = t;code = 116
char = h;code = 104
char = e;code = 101
char = ;code = 32
char = L;code = 76
char = C;code = 67
char = -;code = 45
char = M;code = 77
char = S;code = 83
char = /;code = 47
char = M;code = 77
char = S;code = 83
char = ;code = 32
char = a;code = 97
char = s;code = 115
char = s;code = 115
char = a;code = 97
char = y;code = 121
char = ;code = 32
char = t;code = 116
char = o;code = 111
char = ;code = 32
char = t;code = 116
char = h;code = 104
char = e;code = 101
char = ;code = 32
char = D;code = 68
char = i;code = 105
char = a;code = 97
char = S;code = 83
char = o;code = 111
char = r;code = 114
char = i;code = 105
char = n;code = 110
char = ;code = 32
char = r;code = 114
char = a;code = 97
char = d;code = 100
char = i;code = 105
char = o;code = 111
char = i;code = 105
char = m;code = 109
char = m;code = 109
char = u;code = 117
char = n;code = 110
char = o;code = 111
char = a;code = 97
char = s;code = 115
char = s;code = 115
char = a;code = 97
char = y;code = 121
char = ;code = 32
char = (;code = 40
char = R;code = 82
char = I;code = 73
char = A;code = 65
char = );code = 41
char = ;code = 32
char = a;code = 97
char = n;code = 110
char = d;code = 100
char = ;code = 32
char = a;code = 97
char = ;code = 32
char = r;code = 114
char = e;code = 101
char = c;code = 99
char = e;code = 101
char = n;code = 110
char = t;code = 116
char = l;code = 108
char = y;code = 121
char = ;code = 32
char = r;code = 114
char = e;code = 101
char = -;code = 45
char = s;code = 115
char = t;code = 116
char = a;code = 97
char = n;code = 110
char = d;code = 100
char = a;code = 97
char = r;code = 114
char = d;code = 100
char = i;code = 105
char = s;code = 115
char = e;code = 101
char = d;code = 100
char = ;code = 32
char = v;code = 118
char = e;code = 101
char = r;code = 114
char = s;code = 115
char = i;code = 105
char = o;code = 111
char = n;code = 110
char = ;code = 32
char = o;code = 111
char = f;code = 102
char = ;code = 32
char = a;code = 97
char = n;code = 110
char = ;code = 32
char = a;code = 97
char = u;code = 117
char = t;code = 116
char = o;code = 111
char = m;code = 109
char = a;code = 97
char = t;code = 116
char = e;code = 101
char = d;code = 100
char = ;code = 32
char = e;code = 101
char = l;code = 108
char = e;code = 101
char = c;code = 99
char = t;code = 116
char = r;code = 114
char = o;code = 111
char = c;code = 99
char = h;code = 104
char = e;code = 101
char = m;code = 109
char = i;code = 105
char = l;code = 108
char = u;code = 117
char = m;code = 109
char = i;code = 105
char = n;code = 110
char = e;code = 101
char = s;code = 115
char = c;code = 99
char = e;code = 101
char = n;code = 110
char = t;code = 116
char = ;code = 32
char = i;code = 105
char = m;code = 109
char = m;code = 109
char = u;code = 117
char = n;code = 110
char = o;code = 111
char = a;code = 97
char = s;code = 115
char = s;code = 115
char = a;code = 97
char = y;code = 121
char = ;code = 32
char = (;code = 40
char = E;code = 69
char = C;code = 67
char = L;code = 76
char = I;code = 73
char = A;code = 65
char = );code = 41
char = ;code = 32
char = f;code = 102
char = r;code = 114
char = o;code = 111
char = m;code = 109
char = ;code = 32
char = R;code = 82
char = o;code = 111
char = c;code = 99
char = h;code = 104
char = e;code = 101
char = ;code = 32
char = D;code = 68
char = i;code = 105
char = a;code = 97
char = g;code = 103
char = n;code = 110
char = o;code = 111
char = s;code = 115
char = t;code = 116
char = i;code = 105
char = c;code = 99
char = s;code = 115
char = .;code = 46
char = ;code = 32
char = W;code = 87
char = e;code = 101
char = ;code = 32
char = a;code = 97
char = l;code = 108
char = s;code = 115
char = o;code = 111
char = ;code = 32
char = a;code = 97
char = n;code = 110
char = a;code = 97
char = l;code = 108
char = y;code = 121
char = s;code = 115
char = e;code = 101
char = d;code = 100
char = ;code = 32
char = e;code = 101
char = x;code = 120
char = t;code = 116
char = e;code = 101
char = r;code = 114
char = n;code = 110
char = a;code = 97
char = l;code = 108
char = ;code = 32
char = q;code = 113
char = u;code = 117
char = a;code = 97
char = l;code = 108
char = i;code = 105
char = t;code = 116
char = y;code = 121
char = ;code = 32
char = c;code = 99
char = o;code = 111
char = n;code = 110
char = t;code = 116
char = r;code = 114
char = o;code = 111
char = l;code = 108
char = ;code = 32
char = s;code = 115
char = a;code = 97
char = m;code = 109
char = p;code = 112
char = l;code = 108
char = e;code = 101
char = s;code = 115
char = ;code = 32
char = f;code = 102
char = r;code = 114
char = o;code = 111
char = m;code = 109
char = ;code = 32
char = t;code = 116
char = h;code = 104
char = e;code = 101
char = ;code = 32
char = I;code = 73
char = n;code = 110
char = t;code = 116
char = e;code = 101
char = r;code = 114
char = n;code = 110
char = a;code = 97
char = t;code = 116
char = i;code = 105
char = o;code = 111
char = n;code = 110
char = a;code = 97
char = l;code = 108
char = ;code = 32
char = V;code = 86
char = i;code = 105
char = t;code = 116
char = a;code = 97
char = m;code = 109
char = i;code = 105
char = n;code = 110
char = ;code = 32
char = D;code = 68
char = ;code = 32
char = E;code = 69
char = x;code = 120
char = t;code = 116
char = e;code = 101
char = r;code = 114
char = n;code = 110
char = a;code = 97
char = l;code = 108
char = ;code = 32
char = Q;code = 81
char = u;code = 117
char = a;code = 97
char = l;code = 108
char = i;code = 105
char = t;code = 116
char = y;code = 121
char = ;code = 32
char = A;code = 65
char = s;code = 115
char = s;code = 115
char = e;code = 101
char = s;code = 115
char = s;code = 115
char = m;code = 109
char = e;code = 101
char = n;code = 110
char = t;code = 116
char = ;code = 32
char = S;code = 83
char = c;code = 99
char = h;code = 104
char = e;code = 101
char = m;code = 109
char = e;code = 101
char = ;code = 32
char = (;code = 40
char = D;code = 68
char = E;code = 69
char = Q;code = 81
char = A;code = 65
char = S;code = 83
char = );code = 41
char = ;code = 32
char = f;code = 102
char = o;code = 111
char = r;code = 114
char = ;code = 32
char = c;code = 99
char = o;code = 111
char = m;code = 109
char = p;code = 112
char = a;code = 97
char = r;code = 114
char = i;code = 105
char = s;code = 115
char = o;code = 111
char = n;code = 110
char = ;code = 32
char = w;code = 119
char = i;code = 105
char = t;code = 116
char = h;code = 104
char = ;code = 32
char = o;code = 111
char = t;code = 116
char = h;code = 104
char = e;code = 101
char = r;code = 114
char = ;code = 32
char = p;code = 112
char = a;code = 97
char = r;code = 114
char = t;code = 116
char = i;code = 105
char = c;code = 99
char = i;code = 105
char = p;code = 112
char = a;code = 97
char = t;code = 116
char = i;code = 105
char = n;code = 110
char = g;code = 103
char = ;code = 32
char = l;code = 108
char = a;code = 97
char = b;code = 98
char = o;code = 111
char = r;code = 114
char = a;code = 97
char = t;code = 116
char = o;code = 111
char = r;code = 114
char = i;code = 105
char = e;code = 101
char = s;code = 115
char = ;code = 32
char = u;code = 117
char = s;code = 115
char = i;code = 105
char = n;code = 110
char = g;code = 103
char = ;code = 32
char = L;code = 76
char = C;code = 67
char = -;code = 45
char = M;code = 77
char = S;code = 83
char = .;code = 46
char = ;code = 32
char = T;code = 84
char = h;code = 104
char = e;code = 101
char = ;code = 32
char = m;code = 109
char = e;code = 101
char = t;code = 116
char = h;code = 104
char = o;code = 111
char = d;code = 100
char = ;code = 32
char = w;code = 119
char = a;code = 97
char = s;code = 115
char = ;code = 32
char = l;code = 108
char = i;code = 105
char = n;code = 110
char = e;code = 101
char = a;code = 97
char = r;code = 114
char = ;code = 32
char = f;code = 102
char = r;code = 114
char = o;code = 111
char = m;code = 109
char = ;code = 32
char = 5;code = 53
char = ;code = 32
char = t;code = 116
char = o;code = 111
char = ;code = 32
char = a;code = 97
char = t;code = 116
char = ;code = 32
char = l;code = 108
char = e;code = 101
char = a;code = 97
char = s;code = 115
char = t;code = 116
char = ;code = 32
char = 5;code = 53
char = 5;code = 53
char = 0;code = 48
char = ;code = 32
char = n;code = 110
char = m;code = 109
char = o;code = 111
char = l;code = 108
char = /;code = 47
char = L;code = 76
char = ;code = 32
char = w;code = 119
char = i;code = 105
char = t;code = 116
char = h;code = 104
char = ;code = 32
char = i;code = 105
char = n;code = 110
char = t;code = 116
char = r;code = 114
char = a;code = 97
char = -;code = 45
char = ;code = 32
char = a;code = 97
char = n;code = 110
char = d;code = 100
char = ;code = 32
char = i;code = 105
char = n;code = 110
char = t;code = 116
char = e;code = 101
char = r;code = 114
char = d;code = 100
char = a;code = 97
char = y;code = 121
char = ;code = 32
char = C;code = 67
char = V;code = 86
char = ';code = 39
char = s;code = 115
char = ;code = 32
char = <;code = 60
char = ;code = 32
char = o;code = 111
char = r;code = 114
char = ;code = 32
char = =;code = 61
char = ;code = 32
char = 6;code = 54
char = %;code = 37
char = ;code = 32
char = f;code = 102
char = o;code = 111
char = r;code = 114
char = ;code = 32
char = b;code = 98
char = o;code = 111
char = t;code = 116
char = h;code = 104
char = ;code = 32
char = 2;code = 50
char = 5;code = 53
char = -;code = 45
char = O;code = 79
char = H;code = 72
char = ;code = 32
char = v;code = 118
char = i;code = 105
char = t;code = 116
char = a;code = 97
char = m;code = 109
char = i;code = 105
char = n;code = 110
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char = D;code = 68
char = 3;code = 51
char = ;code = 32
char = a;code = 97
char = n;code = 110
char = d;code = 100
char = ;code = 32
char = 2;code = 50
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char = -;code = 45
char = O;code = 79
char = H;code = 72
char = ;code = 32
char = v;code = 118
char = i;code = 105
char = t;code = 116
char = a;code = 97
char = m;code = 109
char = i;code = 105
char = n;code = 110
char = ;code = 32
char = D;code = 68
char = 2;code = 50
char = .;code = 46
char = ;code = 32
char = R;code = 82
char = e;code = 101
char = c;code = 99
char = o;code = 111
char = v;code = 118
char = e;code = 101
char = r;code = 114
char = i;code = 105
char = e;code = 101
char = s;code = 115
char = ;code = 32
char = r;code = 114
char = a;code = 97
char = n;code = 110
char = g;code = 103
char = e;code = 101
char = d;code = 100
char = ;code = 32
char = b;code = 98
char = e;code = 101
char = t;code = 116
char = w;code = 119
char = e;code = 101
char = e;code = 101
char = n;code = 110
char = ;code = 32
char = 9;code = 57
char = 4;code = 52
char = .;code = 46
char = 9;code = 57
char = ;code = 32
char = a;code = 97
char = n;code = 110
char = d;code = 100
char = ;code = 32
char = 1;code = 49
char = 0;code = 48
char = 6;code = 54
char = .;code = 46
char = 9;code = 57
char = %;code = 37
char = ;code = 32
char = f;code = 102
char = o;code = 111
char = r;code = 114
char = ;code = 32
char = 2;code = 50
char = 5;code = 53
char = -;code = 45
char = O;code = 79
char = H;code = 72
char = ;code = 32
char = v;code = 118
char = i;code = 105
char = t;code = 116
char = a;code = 97
char = m;code = 109
char = i;code = 105
char = n;code = 110
char = ;code = 32
char = D;code = 68
char = 3;code = 51
char = ;code = 32
char = a;code = 97
char = n;code = 110
char = d;code = 100
char = ;code = 32
char = 8;code = 56
char = 2;code = 50
char = .;code = 46
char = 7;code = 55
char = ;code = 32
char = a;code = 97
char = n;code = 110
char = d;code = 100
char = ;code = 32
char = 1;code = 49
char = 0;code = 48
char = 0;code = 48
char = .;code = 46
char = 3;code = 51
char = %;code = 37
char = ;code = 32
char = f;code = 102
char = o;code = 111
char = r;code = 114
char = ;code = 32
char = 2;code = 50
char = 5;code = 53
char = -;code = 45
char = O;code = 79
char = H;code = 72
char = ;code = 32
char = v;code = 118
char = i;code = 105
char = t;code = 116
char = a;code = 97
char = m;code = 109
char = i;code = 105
char = n;code = 110
char = ;code = 32
char = D;code = 68
char = 2;code = 50
char = .;code = 46
char = ;code = 32
char = O;code = 79
char = u;code = 117
char = r;code = 114
char = ;code = 32
char = r;code = 114
char = e;code = 101
char = s;code = 115
char = u;code = 117
char = l;code = 108
char = t;code = 116
char = s;code = 115
char = ;code = 32
char = f;code = 102
char = o;code = 111
char = r;code = 114
char = ;code = 32
char = t;code = 116
char = h;code = 104
char = e;code = 101
char = ;code = 32
char = D;code = 68
char = E;code = 69
char = Q;code = 81
char = A;code = 65
char = S;code = 83
char = ;code = 32
char = s;code = 115
char = e;code = 101
char = r;code = 114
char = u;code = 117
char = m;code = 109
char = ;code = 32
char = p;code = 112
char = o;code = 111
char = o;code = 111
char = l;code = 108
char = s;code = 115
char = ;code = 32
char = a;code = 97
char = v;code = 118
char = e;code = 101
char = r;code = 114
char = a;code = 97
char = g;code = 103
char = e;code = 101
char = d;code = 100
char = ;code = 32
char = -;code = 45
char = 7;code = 55
char = .;code = 46
char = 2;code = 50
char = %;code = 37
char = ;code = 32
char = f;code = 102
char = r;code = 114
char = o;code = 111
char = m;code = 109
char = ;code = 32
char = t;code = 116
char = h;code = 104
char = e;code = 101
char = ;code = 32
char = o;code = 111
char = v;code = 118
char = e;code = 101
char = r;code = 114
char = a;code = 97
char = l;code = 108
char = l;code = 108
char = ;code = 32
char = L;code = 76
char = C;code = 67
char = -;code = 45
char = M;code = 77
char = S;code = 83
char = ;code = 32
char = m;code = 109
char = e;code = 101
char = t;code = 116
char = h;code = 104
char = o;code = 111
char = d;code = 100
char = ;code = 32
char = m;code = 109
char = e;code = 101
char = a;code = 97
char = n;code = 110
char = .;code = 46
char = ;code = 32
char = T;code = 84
char = h;code = 104
char = e;code = 101
char = ;code = 32
char = D;code = 68
char = i;code = 105
char = a;code = 97
char = S;code = 83
char = o;code = 111
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result sql = The plasma 25-OH vitamin D concentration is a reliable biomarker for vitamin D status but assay's variability makes adequate monitoring of vitamin D status difficult. We employed isotope-dilution liquid chromatography (LC) tandem-mass spectrometry (MS/MS) for the measurement of both 25-OH vitamin D3 and 25-OH vitamin D2 in human serum. Hexadeuterium labelled 25-OH vitamin D3 internal standard (IS) was added to calibrators (prepared in phosphate-buffered saline with 60 g/L albumin), controls or patient sera and 25-OH vitamin D metabolites were released from vitamin D binding protein by adding sodium hydroxide prior to protein precipitation by acetonitrile/methanol (9:1, v/v). Subsequent off-line solid-phase extraction was followed by chromatographic separation on a C-18 column using a water/methanol/ammonium acetate gradient. Detection was by Atmospheric Pressure Electrospray Ionisation (AP-EI) followed by selected reaction monitoring. We compared the LC-MS/MS assay to the DiaSorin radioimmunoassay (RIA) and a recently re-standardised version of an automated electrochemiluminescent immunoassay (ECLIA) from Roche Diagnostics. We also analysed external quality control samples from the International Vitamin D External Quality Assessment Scheme (DEQAS) for comparison with other participating laboratories using LC-MS. The method was linear from 5 to at least 550 nmol/L with intra- and interday CV's < or = 6% for both 25-OH vitamin D3 and 25-OH vitamin D2. Recoveries ranged between 94.9 and 106.9% for 25-OH vitamin D3 and 82.7 and 100.3% for 25-OH vitamin D2. Our results for the DEQAS serum pools averaged -7.2% from the overall LC-MS method mean. The DiaSorin RIA agreed well with the LC-MS/MS method (r(2)=0.90; average bias 1.61 nmol/L), the Roche ECLIA considerably disagreed (r(2)=0.58; bias 10.13 nmol/L). This LC-MS/MS method is reliable and robust for the measurement of both 25-OH vitamin D3 and 25-OH vitamin D2 in human serum.

Publication: J. Chromatogr. B Analyt. Technol. Biomed. Life Sci. 2010 May 1 ;878(15-16)::1163-8. :doi: 10.1016/j.jchromb.2010.03.035.
Publication Date: 2010 May 1
Study Author(s): van den Ouweland, Johannes M W;Beijers, Antonius M;Demacker, Pierre N M;van Daal, Henny;
Institution: Department of Clinical Chemistry, Canisius Wilhelmina Hospital, Nijmegen, The Netherlands. [email protected]
Shortcut link to this study:
The plasma 25-OH vitamin D concentration is a reliable biomarker for vitamin D status but assay's variability makes adequate monitoring of vitamin D status difficult. We employed isotope-dilution liquid chromatography (LC) tandem-mass spectrometry (MS/MS) for the measurement of both 25-OH vitamin D3 and 25-OH vitamin D2 in human serum. Hexadeuterium labelled 25-OH vitamin D3 internal standard (IS) was added to calibrators (prepared in phosphate-buffered saline with 60 g/L albumin), controls or patient sera and 25-OH vitamin D metabolites were released from vitamin D binding protein by adding sodium hydroxide prior to protein precipitation by acetonitrile/methanol (9:1, v/v). Subsequent off-line solid-phase extraction was followed by chromatographic separation on a C-18 column using a water/methanol/ammonium acetate gradient. Detection was by Atmospheric Pressure Electrospray Ionisation (AP-EI) followed by selected reaction monitoring. We compared the LC-MS/MS assay to the DiaSorin radioimmunoassay (RIA) and a recently re-standardised version of an automated electrochemiluminescent immunoassay (ECLIA) from Roche Diagnostics. We also analysed external quality control samples from the International Vitamin D External Quality Assessment Scheme (DEQAS) for comparison with other participating laboratories using LC-MS. The method was linear from 5 to at least 550 nmol/L with intra- and interday CV's < or = 6% for both 25-OH vitamin D3 and 25-OH vitamin D2. Recoveries ranged between 94.9 and 106.9% for 25-OH vitamin D3 and 82.7 and 100.3% for 25-OH vitamin D2. Our results for the DEQAS serum pools averaged -7.2% from the overall LC-MS method mean. The DiaSorin RIA agreed well with the LC-MS/MS method (r(2)=0.90; average bias 1.61 nmol/L), the Roche ECLIA considerably disagreed (r(2)=0.58; bias 10.13 nmol/L). This LC-MS/MS method is reliable and robust for the measurement of both 25-OH vitamin D3 and 25-OH vitamin D2 in human serum.
PMID: 20381436

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