All antibiotics marketed in the United States must undergo batch certification by the Food and Drug Administration. It is often assumed that the different brands of a particular antibiotic that pass in vitro batch certification tests have equal in vivo bioavailability. The fallacy of this assumption is clearly supported for commercially available lots of tetracycline hydrochloride that had passed batch certification tests yet had different serial serum concentrations.
(JAMA 232:1349-1351, 1975)
DeSante KA, DiSanto AR, Chodos DJ, Stoll RG. Antibiotic Batch Certification and Bioequivalence. JAMA. 1975;232(13):1349-1351. doi:10.1001/jama.1975.03250130033016