The serum levels of HCG for the same individual with the same body weight fluctuate, but around 10%.
Nobody is saying that if it shows 20% less then Pregnyl - then it's underdosed, but if it's less 5 times - obviously something is wrong.
And this test is widely used in Eastern Europe. And results are consistent.
For example my experience:
12h after 1500IU of pharma Pregnyl always show 45-55 mIU/ml. So we may consider it as a reference.
Before Pregnyl I tested 1 month ago:
AminoAsylum - 9mIU/ml. Obviously underdosed.
ZyHCG - 6 and 7 mIU/ml, underdosed
Ultima hcg - < 5
swiss chems - < 5.
2 years ago Indian ovidac showed 31-35 mIU/ml that is considered also good, but I had different body weight.
I won't list others as it's a long list, but anyway you can see that results 5-10 times less then Pregnyl.
And it's not just fluctuation, that's poor quality.
And regarding bioactivity Jano mentioned in HCG thread that one of the reliable methods is checking testosterone response of Leydig cells after HCG administration, So you may add Total Testosterone(that is also quantitative test) to the HCG Quantitative to make it even more accurate.