What is a common measure of a drug's concentration in the bloodstream?

The measure of a drug's concentration in the bloodstream is typically indicated by plasma concentration. Plasma concentration refers to the amount of the drug present in a specific volume of plasma at a given time. This metric is crucial in pharmacokinetics, as it helps determine the drug's efficacy, safety, and desired therapeutic effect. By measuring the plasma concentration, healthcare providers can assess whether a patient is receiving an adequate dose, is at risk for toxicity, or may need an adjustment in therapy.

Other options, while related to pharmacokinetics, do not directly measure the concentration of the drug in the bloodstream. Volume pertains to the amount of space the drug occupies rather than its concentration. Half-life is a measure of the time it takes for the plasma concentration of a drug to reduce to half its original value, which is crucial for understanding dosing schedules but does not directly quantify drug presence. Total body clearance represents the rate at which the drug is eliminated from the body, reflecting the efficiency of processes like metabolism and excretion, but it is not a direct measure of concentration at a specific time.