When youve tested yourself, did you use the first urination of the day?

Does anyone have any similar experience/knowledge about all of this? And if so, could you please help me understand how accurate these tests are and how the labs test my urine?
The lab will first perform an Immunoassay screen with likely a 50 ng/ml cutoff limit, which is the same as most home THC tests. Because home tests function on the same principle as the I/A screens, one is no more accurate than the other.

If the sample tests negative on the initial I/A, the testing process is over then as a "pass". If the sample tests positive, it will undergo a confirmation test using the GC/MS, which is more accurate than the I/A.