Q&A column

Editor: Frederick L. Kiechle, MD, PhD

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Q. Is there a CAP guideline that recommends that patients stop taking drugs that may interfere with a blood or urine test before providing a specimen?

A.August 2023—There is no CAP guideline and no other recommendation that we know of pertaining to stopping drugs that may interfere with laboratory testing prior to specimen collection. This is an important topic for all laboratory testing, not just for toxicology. There are many factors to consider when determining possible drug interference, including what drugs the patient is taking, what testing has been requested, and what testing modality is being used.

The laboratory must first identify what prescription and over-the-counter medications and herbal supplements the patient is taking and whether it is safe to pause their use.¹ Many patients may have adverse outcomes if they stop taking their medication. In general, patients should not stop taking their anti­depressants.²

If a medication can be safely stopped, the half-life helps inform when it should be stopped prior to testing. The body fluid (i.e. urine, serum) being tested determines whether the plasma half-life or elimination half-life should be used. In general, a drug will be sufficiently eliminated after five half-lives.

The testing modality being used must also be considered. Immunoassays are more susceptible to interference and cross-reactivity from structurally similar compounds, while other assays are less susceptible.² For example, bupropion may cause a false-positive result in some amphetamine immunoassays. Therefore, immunoassays are useful for initial screening of a sample but should be followed up with confirmation testing, often using mass spectrometry, to help avoid false-positive results. That said, ion suppression, isomers, and isobaric compounds may interfere with mass spectrometry assays. Using multiple testing modalities may help overcome the issue of drug interference in specific modalities and provide accurate results.

Physiological and clinical factors, including the presence of medication metabolites, must also be considered when assessing whether a medication may affect laboratory testing.³ Resources such as Effects of Drugs on Clinical Laboratory Tests⁴ and Resolving Erroneous Reports in Toxicology and Therapeutic Drug Monitoring: A Comprehensive Guide² provide a starting point for addressing this issue, but due to the evolution of testing methods and complexity of drug-drug interactions, a thorough literature search is often needed.

1. Dasgupta A. Effects of Herbal Supplements on Clinical Laboratory Test Results. De Gruyter; 2011. Patient Safety; vol 2.
2. Dasgupta A. Resolving Erroneous Reports in Toxicology and Therapeutic Drug Monitoring: A Comprehensive Guide. John Wiley & Sons; 2012.
3. Guder WG, Narayanan S, Wisser H, Zawta B. Samples: From the Patient to the Laboratory. 3rd rev ed. Wiley-VCH; 2003.
4. Young DS. Effects of Drugs on Clinical Laboratory Tests. 5th ed. AACC Press; 2000.

Miranda D. Chimzar, MD
Post-graduate Year-three Pathology Resident
Department of Pathology
University of Utah and ARUP Laboratories
Salt Lake City, Utah