Utilizing the THC/Creatinine Ratio (part 2)

Dr. Joseph E. Graas, Scientific Director
Dr. Edward Moore, Medical Director

When abnormally large quantities of fluids are consumed, the urine becomes diluted and the creatinine levels are substantially reduced, as well as the levels of the drugs and their metabolites.  This fluctuation in the amount of creatinine in each sample allows for a correction of the drug excreted in that same sample.  The ratio of the THC to creatinine should be the same, independent of the dilution of the sample due to water intake.  If the patient does smoke marijuana, then the value for THC or more specifically the ratio of THC/Creatinine, when measured on different days, should be useful in determining continued smoking of marijuana.

As an example, if a patient gives a sample on Intake (A), and has reported chronic use, it is expected that the sample will be positive for THC.  For this patient, there will be a sample collected on Intake (A), and once a week for the following four weeks (B, C, D, E), for a total of five samples collected.

The THC/Creatinine Ratio is calculated as follows: (THC Value/Creatinine)*100

The way to monitor if the patient has ceased use is to monitor the ratio, not the actual values.  In the example, it is expected that sample A will be positive for THC.  The ratio for sample A is the baseline.  If the person has discontinued use, it is expected that the sample will likely continue to be positive for THC, but that the ratio will decrease.  In the example, the ratio decreases between samples A, B and C.  However, a noticeable spike (or increase) occurs in the ratio between testing events C and D, indicating new THC use between these two testing events.  The ratio for sample D would be the new baseline, and you would expect a decrease in the ratio from that testing event on as the drug metabolizes.  But, in the example, there is a second spike in the ratio between samples D and E.  This would indicate, again, new use between testing events D and E.

Further, in samples D and E, the THC values are 24 and 18.  In almost any state, these screening values would be considered negative.  (Most clients honor a cutoff level of 50 in screening for THC.) In this example, the patient’s counselor notices the discrepancy between a negative result for THC, a creatinine level that is on the lower side (but still normal), and a THC/Creatinine ratio that is numerically higher than event C.  Upon discussing with the patient, the patient admits to consuming an excess amount of fluids prior to submitting the test.

The intent of the ratio is to monitor ongoing positive results over time as the drug metabolizes out of the body, to verify cessation or ongoing use, and to help in the treatment of patients.