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Specimen Selection and Timing of Collection To calculate a normalized concentration, use the following formula: It is best if the same creatinine and THC methods are utilized for serial samples collected from the same patient. To determine a creatinine-normalized THC concentration, creatinine testing should be ordered or performed at the same time a urine specimen is collected for THC testing. Many quantitative clinical assays for detection of THC and related metabolites (including ARUP’s) do not routinely include creatinine measurement. To demonstrate elimination of THCA (decreasing concentration) or new use of marijuana (increasing concentration of THCA), an appropriate testing interval is no more than once per week. If a patient has abstained from new use of marijuana, the concentrations in urine of the creatinine-normalized delta-9-tetrahydrocannabinol (THC) metabolite, THC acid (THCA), should decrease over time. Hair specimens, meconium, and umbilical cord tissue are useful for demonstrating chronic exposure/use.Ĭreatinine normalization may be useful to evaluate whether a patient has abstained from new use of marijuana. Blood specimens are also appropriate for patients on dialysis, for suspected cases of malabsorption (eg, gastric bypass), and for evaluating other aspects of an individual patient’s pharmacokinetics. Blood collection is an observed procedure, which lowers the likelihood of specimen adulteration or substitution. Blood (serum or plasma) is the preferred specimen for correlating signs and symptoms with drug concentrations in a real-time acute setting. However, not all drugs are detectable in saliva, and both urine and saliva are susceptible to adulteration or substitution by the donor. Drug concentrations and the time course for detection in saliva more closely approximate concentrations and the detection window in blood than in urine. Saliva (oral fluid) is also noninvasive to collect but is associated with higher costs. Urine is preferred because its collection is noninvasive and inexpensive, and drugs and their metabolites tend to concentrate in the urine over time.