A late specimen can establish that a compound was still measurable at collection. Explaining why it was measurable requires a separate analysis. A case report on oseltamivir illustrates the difference when the parent drug and its active metabolite are measured together.
In serum collected 154 hours after the last dose, oseltamivir was below the assay’s 0.5 ng/mL detection limit, while its active metabolite, Ro 64-0802, was measured at 1.30 ng/mL. Both compounds were below the detection limit in the cerebrospinal fluid specimen. A below-limit result describes the assay’s ability to detect an analyte in that specimen.
The authors modeled slower conversion to the metabolite and slower elimination of the metabolite. Both could account for the late result under their assumptions. In a toxicology report, those simulations belong alongside the actual result as possible explanations. One late concentration cannot independently resolve which process was responsible or reconstruct an earlier concentration in the brain.
Morimoto and colleagues, BMC Neurology (2015). Journal article · Full text.