Percent of active in the dosage form. If the bottle says there is 150mg of X in each tablet, then there needs to be 100.0 +/-2.0%, or 148mg to 152 mg in there when you test them, for example (assuming that was the criteria for that drug). The impurities are often a percentage of the active ingredient (Y is 0.5% of 150mg), but sometimes they are calculated differently.
In quality control situations, the inactive ingredients are never tested directly - if a tablet is to weigh 250mg with 150mg of active, then it’s assumed the other 100mg are inactive ingredients and since they have no pharmacological effects anyways, there’s no reason to measure them. Impurities are tested because they have pharmacological effects, as are degradation products (which you don’t want to have, because that means your 150mg is slowly decaying to something else, which has been the subject of many threads here about product stability testing).
I kind of realise that this isn’t a very helpful fact in a thread mostly about blood levels/pharmacokinetics and the effectiveness of generics. I blame it on how early it was when I posted, and how I really should have been studying for this morning’s final exam!
I have very little background in phama-kin/toxicology and have never worked in a clinical trial situation. I was an HPLC/GC/UV-vis/IR/Dissolution monkey.
If anyone cares about the tangent I’ve started on, here’s the USP monograph(PDF!) for acetaminophen (Tylenol) capsules and tablets. It’s usually the starting point for analytical testing criteria and method development, but they are kind of based on older technology than what most labs currently own and so methods are usually tweaked somewhat to give better, more repeatable results.
Note the USP has a 100% +/- 10% of labelled dosage range, but IME, the actual manufacturing companies will maintain a much stricter range. Uniformity of Dosage just tests several capsules/tablets to make sure they are all conforming to the criteria.
In simulated dissolution (literally dropping the capsule in solution and measuring how much dissolves over time), the criteria and testing conditions vary for capsules and tablets because of their coatings but are in the range of 75-80% release in 30-45 mins.
All acetaminophen products, whether generic or brand name, need to meet at least these criteria at minimum in order to obtain approval from the FDA.