The actual delivered dose, aerodynamic particle size distribution and lung deposition of inhalers depend on three correlating factors: patients, prescriptions and inhaler devices. Due to the characteristics of inhalers' forms, the current in vitro test methods regulated by Pharmacopoeia as well as the individual different situations, the correlation between in vitro and in vivo is weak and IVIVC studies become necessary. IVIVC conducts in vitro testings by simulating human breath.  

There are individual differences between different patients, and the actual breathing pattern and inspiratory flow of the patient during inhalation are not consistent with the set flow in the in vitro test, which affects the drug distribution in the lungs and leads to inconsistent results in the in vitro test.