One might initially think that serum would be the preferred sample for DOA testing. After all, serum is a highly-controlled, homeostatic fluid that reflects the exact metabolic state of the patient. Furthermore, it's easy to substitute or tamper with a urine sample, since individuals being tested collect the urine themselves. It would be much harder to tamper with a serum sample. So why don't we use serum for routine DOA testing?
The reason is that urine actually gives us a better window into the patient's history. Serum will contain traces of drugs that have been used but the liver and other tissues quickly clear the blood of drugs. Although each drug has a different half-life or kinetic in the blood, most are cleared fairly rapidly, within hours.
Urine, on the other hand, tends to concentrate drugs over time. This is due to the simple fact that urine represents a small amount of volume compared to the total fluid in the body. As drugs are cleared by the kidneys, the urine becomes more and more concentrated with the drugs that were once present in the serum.
As an example, consider the opiate codeine. In the serum, an appropriate concentration of codeine would be around 13-35 ng/mL. However, due to the concentrating effect of urine, we don't even call a patient's urine positive for codeine until the concentration reaches 150 ng/mL. The concentration in urine is more than 10-times that in serum!
Other advantages to urine as samples for DOA testing are:
- The samples are readily preserved by freezing.
- Drugs are stable in urine (generally no cells present to further metabolize the drugs).
- It is easier to obtain (although this also means it is easier to tamper with or adulterate).