The labs that are accredited by the World Anti-Doping Agency (WADA) to screen athletes blood and urine samples use techniques that are very sensitive — with capability to detect banned substances down to 2 nanograms per milliliter of urine (i.e. 2 parts of substance in 1,000,000,000 parts of urine).
Doses (total intake) of low levels of banned substance (in the microgram range) can generate such levels in an athlete's urine. As a result, supplements need to be tested for the presence of banned substances at sensitivity levels in the nanogram per gram or nanogram per milliliter range (as an athlete may ingest many tens of grams of supplement). Such low detection levels are beyond the "normal" QC requirements of pharmaceutical and food industries.
In supplements tested through LGC and the INFORMED-CHOICE program, the Level of Detection is 10 ng/g for steroids and 100 ng/g for stimulants.