Gastric fluid, simulated, TS (Test Solution) is a solution with pepsin that is used for dissolution testing. It is described in the Test Solutions section of the United States Pharmacopeia. It is one of the original dissolution media used to simulate the conditions of the stomach. The more commonly used solution without enzyme is referred to as USP-SGF (sp), where "sp" stands for sine pepsin (without pepsin).
USP-SGF (sp) – is a solution of hydrochloric acid with sodium chloride. It has a pH of 1.2 with an osmolarity of approximately 180mOsmol/kg.

USP-SGF (sp) is NOT FaSSGF
Biorelevant FaSSGF medium and the corresponding FaSSGF buffer used for the preparation of FaSSGF differs significantly from USP-SGF (sp) buffer.
•    FaSSGF biorelevant medium contains surfactants that mimic the gastric environment in the fasted state. These surfactants lower the surface tension of the medium to a physiological level thereby encouraging wetting of drug products with poorly soluble drugs. USP-SGF (sp) buffer does not contain these surfactants and, therefore, does not have the appropriate surface tension.
•    FaSSGF biorelevant buffer is the buffer used to prepare FaSSGF (in combination with 3F Powder®). It is designed to simulate the aqueous environment (in terms of average pH) of fasted state gastric fluid more closely than USP-SGF (sp). FaSSGF has a pH of 1.6 which is closer to the average gastric pH compared with pH1.2 of USP-SGF (sp).