In Spanish the word dosage does not exist (see RAE), it is an extended adaptation in the medical jargon of the English "dosage". The correct thing would be to talk about determination / measurement / concentration (in plasma, serum, urine, etc). On the other hand, the expressions relating to "dose" (indicated or administered) are dosing or dosing.
* Only one "like" per meaning and day, the more "likes" the meaning will appear higher in the list
