difference between mesurement by GC/MS and by a kit

[françois]

Hello everyone,
for a project I m used to measure plasma total cholesterol by GC-MS.
However, when I compare the measurement result obtained (from the same sample) by GC/MS or by an enzimatic kit, I show that plasma cholesterol is increased by three in the enzimatic kit.
one of my hypothesis is that the enzimatic kit (cholesterol RTU, Biomerieux) is less specific than GC/MS ans detect cholesterol and derivatives.

Have you ever mad this observation after a measurement of cholesterol or other molecules ?

thanks in advance !

François MOREAU, PhD student
L'institut du thorax
Unité Inserm UMR 1087 / CNRS UMR 6291
IRS, 8 Quai Moncousu
BP 70721
44007 Nantes cedex 1

[James_Ball]

I am working on Vitamin D by LCMSMS and from researching the literature I am finding that the LCMSMS gives higher values for total Vitamin D because Vitamin D2 and Vitamin D3 do not respond equally on the enzymatic tests and will give improper readings depending on the ratio of D2 to D3 compared to the values determined by chromatography.

The enzyme would need to be very specific to only react to cholesterol and not any of the compounds with similar functional groups.