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Mayo Clinic LabsWeeding Through the Information: Interpreting Laboratory Tests to Determine New vs. Residual Use of Marijuana

Mayo Clinic has developed a test that measures both the carboxy-THC and the urine creatinine, facilitating the use of decision ratios to determine new vs. residual use.

Understanding the difference between new and residual marijuana use is crucial for physicians in various settings, from addiction management programs to employment screenings. Dr. Paul Jannetto of the Mayo Clinic provides insights into how laboratory tests, particularly the carboxy-THC to creatinine ratio, can help in making this distinction.

Key Points:
  • Basic Facts: Marijuana is the most commonly used addictive drug in the US, with THC as its primary psychoactive component.
  • Decision Ratios: Using urine creatinine normalized carboxy-THC concentrations at two time points and calculating a decision ratio can help distinguish between new and residual use with high accuracy.
  • Case Study: A clinical case study demonstrated the application of these decision ratios, confirming their utility in real-world scenarios.
  • Physician Perspectives: Understanding the nuances of marijuana testing is essential for accurate diagnosis and treatment, especially in settings like addiction management and legal cases.
Additional Points:
  • Pharmacodynamics: The amount of THC in marijuana has been steadily increasing, affecting its short- and long-term health impacts.
  • Testing Methods: Besides immunoassays, gas chromatography or liquid chromatography tandem mass spectrometry are also used for more definitive results.
  • False Positives/Negatives: Immunoassays can sometimes yield false positives due to cross-reactions with other substances like antiviral medications.

More on Marijuana/Cannabis…

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