Pharmacokinetics. The change in the distribution and chemical properties of a drug after administration to a human or animal is termed pharmacokinetics. Pharmacodynamics refers to the time course of effects of the drug after administration. There is unfortunately a considerable lack of correlation between these two measures. The effects of drugs often subside long before drug levels have diminished. Both Pharmacokinetics and Pharmacodynamics give unique and essential insight into drug mechanisms.
Data Sources - Analytical Chemistry. Scientists among scientists devote considerable attention to the methods used to obtain data, and scrutinize the data before drawing conclusions. In contrast, forensic laboratories, the courts, and the legal community in general seems to have little patience and expertise for examining these aspects, but readily accepts data as valid, racing off to various interpretations.
Blood levels of Drugs. Blood samples are obtained from living persons or post-mortem. Some drugs are susceptible to Drug Redistribution as part of the post-mortem process. Serum Drug Levels may be significantly different from Plasma Drug Levels, altering interpretation. Some drugs are highly bound to proteins in blood.
Urine levels of Drugs. Urine has more shortcomings than blood levels for technical reasons and also interpretive reasons. Whereas blood levels are always changing and often reflect the bioactive concentration of drug, urine levels (of the drug or its metabolite) give a retrospective estimate of the amount of drug elimination since last bladder voiding. The volume of urine is a relevant parameter also which affects the detected drug concentration, and is affected by the hydration state of the subject. For these reasons, drug tests performed on urine are typically non-quantitative, and only intended to give an indication whether a drug has or has not been used "recently".