hi , i am not disputing or agreeing with what is being stated here, as frankly i don't have the relevant knowledge to do either.
But one thing i have read is the reason that igf is chosen over gh serum when a single blood test is being used to look at growth hormone deficiencies/over production etc, because with gh for the vast majority of the time your serum level sits at virtually 0 until you have a GH pulse, so for the most part everybody would simply give a result near 0 even with perfectly normal GH secretion function.
as IGF levels are stable and dependent on GH secretion , this is why IGF is the preferred test because if your GH secretion is not working properly your IGF levels will generally reflect this.
I'm sure when testing more thoroughly they generally monitor a patient over a 24 hr period , to measure their GH secretion over a prolonged period.