Can they test the substance again independently?

A defendant can typically seek independent testing of the seized substance, through a motion and an approved laboratory.

When it's worth pursuing: where the identity of the substance is genuinely in question, where the weight sits near a threshold that changes the charge, where the State's result appears inconsistent with other evidence, or where the analysis method has known limitations for that substance.

Practical constraints. Sufficient sample has to remain, and some testing is consumptive. It costs money. And it takes time that has to fit the case schedule.

What often matters more than a second test is examining the first one — the analyst's qualifications, the instrument's calibration and maintenance, the method used, whether controls were run, and whether the lab's own protocols were followed. The underlying data behind a one-page report frequently says more than the report does.