IGF-1 testing (along w/ some mass specs) is by far the most accurate.
However, there are very good reasons to use blood serum testing.
You pituitary, assuming you are a healthy younger man, produces GH in bursts. This fact is common knowledge. Thus, endogenous GH is going to fluctuate markedly.
Conversely, IGF-1 is steadily and consistently elevated concomitantly with exogenous GH use.
Why do people use b serum? Hypothetically, lets say you have 100 kits coming in from China. 60 are on back-order IE have already been purchased in advanced. You simply do not have the time to wait 6 weeks for an IGF-1 test. You need to know now, preferably before your kits land from China.
That is why you need immediate data. Lets say you get a 17.3 ng lab result. That translates to the bottles being 8 iu's each. You can then be upfront about potency.
Now in regards to purity, that is a question that must be addressed via Mass specs.
Mass specs are expensive.
However, given all the shitty GH coming from China, as a consumer I would want to see both mass specs and b serum results before opening my wallet. Very few srcs have this data.