Creatinine and New Use of Marijuana
Marijuana (THC) can persist in the urine long after use - from two days for the occasional user to six weeks for the chronic user. Metabolism rate and body fat are also factors in the length of time THC may be detected in urine. Whereas the THC levels in the body tissues will decline steadily, actual urine levels may fluctuate due to varying degrees of urine hydration/dilution. There is often a need to determine if a current positive THC result reflects new marijuana use or previous use. In principle, the urine THC level should decrease over time following the last use, however, the level also depends upon the urine concentration. In order to handle this concentration variable, the THC level is divided by the creatinine level (CR). The THC/CR ratio should decrease over time when there is no new use. The rule-of-thumb is that when comparing two results, the THC/CR ratio should decrease by 50% every two to ten days depending on the individual. A light or infrequent user will decrease faster than a heavy or frequent user.

When comparing levels, it is important to compare results derived from the same test method, either compare EMIT values with EMIT values, or compare GC/MS values with GC/MS values.

The following table shows data from one donor who provided three specimens on three different dates.

Collection Date:02-02-0802-10-0802-20-08
THC level350450180
Creatinine level52150180
THC/CR ratio6.73.09.0

By following the THC level alone, one might conclude that there was new drug use between specimens A and B since the THC level increased. However, the THC/CR ratio decreased from 6.7 to 3.0 indicating no new use. In contrast, the THC/CR ratio increased from 3.0 to 9.0 indicating new marijuana use between specimens B and C.