Unbound drug concentration
Unbound concentration is the fraction of total drug concentration that is free of binding in the sampled matrix.
Unbound concentration is the fraction of total drug concentration that is free of binding in the sampled matrix (FDA. Physiologically Based Pharmacokinetic Analyses: Format and Content. 2018., ICH. S3A: The Assessment of Systemic Exposure in Toxicity Studies. 1994.).
If the fraction unbound is 0.05 and total plasma concentration is 20 mg/L, unbound plasma concentration is 1 mg/L at that condition. The conversion assumes the fraction is known for the same matrix and concentration range. Transport, tissue binding and metabolism can make the relationship to unbound concentration at a tissue target more complex.