Utilizing the THC/Creatinine Ratio (part 1)
Dr. Joseph E. Graas, Scientific Director
Dr. Edward Moore, Medical Director
Marijuana is a fat soluble drug with a long plasma half life. This leads to confusion in interpretation when urine samples are collected at different times. If a patient ceased using marijuana, one would expect that the values for THC would drop as different samples are collected. However, when the urine is dilute or concentrated, the THC values will become higher or lower dependent on the dilution of the urine.
The absence of THC in a urine sample is very good evidence that a person is not continuing the use of the drug. To achieve this absence in a chronic marijuana user can take up to 30 days after the use of the drug has been discontinued. During this period of time, monitoring the urine values (levels) for THC concentration can lead to confusing results, and often times the values obtained are quite unusable.
In some samples, the variability of the urine concentration results can range from being very concentrated (such as the first void in the morning) to very dilute (when a person drinks a lot of water or other beverages). Lifestyle can also affect the urine composition. The main issue with THC is that it is extremely insoluble in water samples and very soluble in organic media, which is to say, the drug is stored in the human body in fat tissue. This storage is a function of the length of use of the drug, the amount of the dose and the frequency of daily use. The storage of the drug under these chronic conditions results in a slow elimination. When the variability of the urine concentration is also factored in, the resulting elimination of the drug can fluctuate quite a bite from sample to sample and day to day.
To compensate for this fluctuation in urine concentration, a creatinine value can be used. Creatinine values are utilized in a qualitative sense to state that urine samples have either been diluted with fluids (most commonly water) or that excess beverage consumption has resulted in voiding a diluted sample. However, by using the analytical value determined for creatinine and adjusting the units of measure so that a comparison between the THC and creatinine values can be made, it is possible to obtain an informed opinion as to weather or not a serial urine sample is higher or lower in THC.
Creatinine is a natural compound that is produced in the body and excreted in the urine in relatively constant amounts on a daily basis. The amount excreted by each individual is about 1 to 1.2 grams/day. Because creatinine is excreted from the body at a constant rate, there are expected creatinine values in normal human urine. However, the amount found for creatinine in each voided specimen when added up will result in a value of 1 to 1.2 grams. Thus, a diluted sample will have less creatinine and less THC in it. The ratio will then correct for this variability in the urine volume.
Part 2 of this article will appear in the February issue of Toxicology Times.