I should’ve said immunoassay, which is
the most common method for measuring IGF.
Chemiluminescent immunoassays (CLIA) grab IGF-1, “light up”, and are read by machine to determine IGF-1 levels.
The problem is some people produce variants of IGF-1 that are active, but aren’t detected by CLIA. In other cases, variants are produced that are inactive, but CLIA measures them as if they are functional IGF.
So CLIA counts active normal IGF (also called “Wild Type” or WT) and IGF variants whether inactive or active, as the same thing. It also misses some variants altogether. There’s no mention of variants in a Labcorp or any other “old fashioned” immunoassay test.
Quest says its LCMS test method doesn’t have this issue. It sees all variants, though only counts “wild type” ie “normal” for its IGF-1 result and Z score.
If they detect variants, it’s mentioned in the notes, along with estimated levels of them.
Not detecting variants has been a problem when patients:
- have normal GH but unexplained low IGF,
- or they’re receiving rHGH treatment but IGF is much lower than expected
- or IGF looks “normal”, on or off rHGH, using a CLIA test like Labcorp, but they have symptoms of low IGF like high body fat and difficulty putting on muscle. It’s potentially because the CLIA is including INACTIVE IGF variants.
A much better understanding of how widespread this problem is comes from a recent study looking at a quarter million patients, and finding .5% of them had significantly wrong Z and IGF because of these IGF variants not being detected.
(Remember “WT” means “wild type”, in other words, “normal” IGF-1.)
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Here the report found that on average, Z scores are much lower in those with variants, and when active variants were added back in, they returned to normal.
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Here the report notes Quest now reports IGF-1 variants, and suggests doctors need to start paying attention to this:
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TLDR: Quest / LCMS method of testing IGF-1 is the only truly accurate method available for the 0.5% of the population with IGF-1 variants. That percentage may be a significant undercount of the true number with variants.