In general, the concentration of the parent drug in urine exceeds that of its metabolite(s). For this reason, it is ideal to target the parent drug rather than metabolites in drug testing. However, this is not always possible because some medications are rapidly metabolized. For example, heroin and cocaine can only be detected in urine for a few hours after use, but their metabolites can be detected for several days.
It takes approximately six half-lives to eliminate 99% of any drug. The half-life of a drug is the time it takes for 50% of the amount of drug consumed to be metabolized and eliminated. After two half-lives, the amount of drug remaining in the body is 25%; after three half-lives, 12.5%; after four half-lives, 6.25%, etc. When the concentration of the drug is reduced to about 5%, the concentration is considered negligible. As discussed in the following pages, many factors can influence the half-life.
Some drugs have metabolites that can also be prescription drug formulations. Examples are:
- Methamphetamine → Amphetamine
- Codeine → Morphine metabolite
- Hydrocodone → Hydromorphone
- Codeine → Hydrocodone
- Oxycodone → Oxymorphone
Drug concentrations seen in confirmation testing depend on many factors. Many physicians would like to correlate drug concentrations with the amount consumed. But due to a multitude of factors, this is impossible. While serum levels of drugs are related to dose, urine concentrations are not. This is because of the concentrating effect in urine and the fact that urine is not a hemodynamically controlled fluid (its concentration depends on how much one drinks, which fluctuates greatly).
