The relevant limits of detection for analysis at the manufacturing stage are much smaller than those conventionally addressed by "standard" quality control procedures in the supplements industry.
In general, the necessary levels of detection used in anti-doping screening for athletes are measured at the part per billion level (i.e. nanogram per ml). However, the supplement manufacturing industry would generally use parts per million (i.e. microgram per ml or microgram per gram) for detection of general contaminants for quality control purposes. This means that an athlete's urine or blood test is much more sensitive and is more likely to detect the presence of a prohibited substance than when a supplement is tested during the manufacturing process using conventional methods, with parts per million detection limits.
The concentration levels expressed in terms of a standard unit (i.e. a gram or ml in this case) are not as relevant as the total intake of a supplement within a daily dose, which may vary from a few grams to perhaps over 100 grams depending on the product. For example, an athlete would consume a greater volume of the supplement taking one serving of creatine than they would in taking one vitamin tablet. The more one takes, the higher the risk of having traces of a prohibited contaminant in your system.
The levels and limits of analytical detection have profound implications on supplement quality assurance program, demanding testing sensitivities that can only be met by highly specialized analytical laboratories, working to ISO 17025 standards. It is extremely uncommon for the manufacturing facilities used by supplement companies to have the capability to undertake such trace analysis, concentrating instead on the verification of label listings of specified contents. In most cases, supplement manufacturers will test products for other contaminants such as toxic substances, heavy metals, etc. and not necessarily for those substances that may give rise to a positive drug test for a professional athlete, such as nandrolone or ephedrine.