timetoabsorb

Timetoabsorb, abbreviated t_abs, is a pharmacokinetic concept that describes the interval from the administration of a substance to the onset of its systemic absorption. It denotes when the substance first begins to enter the bloodstream, or when a measurable systemic concentration is reached, depending on the definition used in a study. This metric is related to, but distinct from, the time to maximum concentration (tmax), which reflects the combined effects of absorption and distribution on observed plasma levels.

Measurement and interpretation of t_abs rely on concentration-time data collected after dosing. In practice, t_abs is influenced by the route of administration and the formulation, and may be inferred from early data points on a plasma concentration-time curve. For oral dosing, t_abs can be affected by gastric emptying, intestinal transit, and the release characteristics of the dosage form, and is often shorter than tmax. Routes with rapid absorption or immediate-release formulations tend to yield t_abs values closer to tmax, while extended-release or poorly absorbed substances may show a larger gap between t_abs and tmax.

Factors affecting timetoabsorb include route of administration (oral, subcutaneous, transdermal, inhalation), formulation properties (solubility, particle size, excipients, release rate), and physiological conditions (gastric pH, blood flow, food intake, concomitant medications). t_abs can vary substantially between individuals and across occasions, limiting its predictability for individual patients.

See also: bioavailability, tmax, onset of action, pharmacokinetics.