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Sources and method

By The reference desk · Fact-checked against the cited sources

Published · Sources checked Oct 4, 2026

Short answer

Every figure on this site comes from a federal testing rule, a federal statute or a peer-reviewed study. Each carries the number of its source, and the list says what part of the source was read and when.

Where figures come from

Only primary sources: the federal workplace testing rules for urine and oral fluid,[1][2] the federal hemp statute,[11] and peer-reviewed studies. Four of the seven studies come from the intramural research program of the National Institute on Drug Abuse, part of the National Institutes of Health (NIH).[4][5][7][8] A clinical review in Mayo Clinic Proceedings supplies the commonly quoted urine ranges.[3] No figure comes from a test-kit seller, a forum or another website’s summary.

What was read

Each entry in the numbered list says what was checked: the full text of the federal rules’ cutoff tables and THC discussion,[1][2] the statute text,[11] and the published abstracts of the studies.[4][5][6][7][8][9][10] Source 3 is marked “Editor check of the figures pending” until its tables have been read in full; the urine ranges that depend on it carry that mark until then.[3]

How the table is built

  1. Each cell is a range reported by a cited source, for one test and one pattern of use.
  2. The patterns are approximate. Studies define “occasional”, “frequent” and “chronic” users in their own ways, so each cell follows its study’s wording as closely as a short table allows.
  3. Where the research does not support a range, the cell says “Not well studied”. Hair after a single use says “Often not detected”, because the hair study found most light users negative.[9]

How the estimate form works

The form takes three answers: the test, how often, and the days since last use. It looks up that cell and compares your day with its range. A day before the low end is “inside”, a day between the ends (rounded out to whole days) is “at the edge”, and a day after the high end is “past”. An open-ended range (“30 days or more”) has no “past”. The form asks nothing about body size or anything else personal, because the cited studies do not support a personal estimate. It saves nothing and sets no cookie.

Limits

Most studies used smoked cannabis and small groups, and several held people on research units where use could be ruled out. Windows depend on the dose, the cutoff and the person. Nothing here predicts one person’s result; only a lab test can.

What this site will not publish

No advice on changing a test result, no product links for tests, and no promise about anyone’s result. Time since last use is the only thing the studies show lowering what a test finds.[4][7]

Questions

Who checks the figures?

An Editor reads each source and records what was read and when. Guides are fact-checked against the cited sources. No guide is presented as reviewed by a doctor until a named, credentialed reviewer has signed it off.

How does the estimate form decide inside, edge or past?

It compares the day you enter with the published window: before its low end is inside, between its ends is the edge, after its high end is past.

Why do some cells say not well studied?

Because no cited source reports a reliable window for that test and pattern of use.

Can I report a wrong figure?

Yes. Email the page address and the source number through the Contact page, and an Editor rereads the source.

Sources

Sources checked Oct 4, 2026. Numbers in the text link here.

  1. Substance Abuse and Mental Health Services Administration. Mandatory Guidelines for Federal Workplace Drug Testing Programs. Federal Register 82 FR 7920, January 23, 2017. www.federalregister.gov/d/2017-00979Checked Oct 4, 2026 against the full text: the cutoff table.
  2. Substance Abuse and Mental Health Services Administration. Mandatory Guidelines for Federal Workplace Drug Testing Programs: Oral Fluid. Federal Register 84 FR 57554, October 25, 2019. www.federalregister.gov/d/2019-22684Checked Oct 4, 2026 against the full text: the cutoff table and the THC discussion.
  3. Moeller KE, Kissack JC, Atayee RS, Lee KC. Clinical Interpretation of Urine Drug Tests: What Clinicians Need to Know About Urine Drug Screens. Mayo Clin Proc. 2017;92(5):774-796. doi.org/10.1016/j.mayocp.2016.12.007Checked Oct 4, 2026 against the citation. Editor check of the figures pending.
  4. Lowe RH, Abraham TT, Darwin WD, Herning R, Cadet JL, Huestis MA. Extended urinary Delta9-tetrahydrocannabinol excretion in chronic cannabis users precludes use as a biomarker of new drug exposure. Drug Alcohol Depend. 2009;105(1-2):24-32. doi.org/10.1016/j.drugalcdep.2009.05.027Checked Oct 4, 2026 against the abstract.
  5. Desrosiers NA, Himes SK, Scheidweiler KB, Concheiro-Guisan M, Gorelick DA, Huestis MA. Phase I and II cannabinoid disposition in blood and plasma of occasional and frequent smokers following controlled smoked cannabis. Clin Chem. 2014;60(4):631-643. doi.org/10.1373/clinchem.2013.216507Checked Oct 4, 2026 against the abstract.
  6. Odell MS, Frei MY, Gerostamoulos D, Chu M, Lubman DI. Residual cannabis levels in blood, urine and oral fluid following heavy cannabis use. Forensic Sci Int. 2015;249:173-180. doi.org/10.1016/j.forsciint.2015.01.026Checked Oct 4, 2026 against the abstract.
  7. Bergamaschi MM, Karschner EL, Goodwin RS, Scheidweiler KB, Hirvonen J, Queiroz RH, Huestis MA. Impact of prolonged cannabinoid excretion in chronic daily cannabis smokers' blood on per se drugged driving laws. Clin Chem. 2013;59(3):519-526. doi.org/10.1373/clinchem.2012.195503Checked Oct 4, 2026 against the abstract.
  8. Swortwood MJ, Newmeyer MN, Andersson M, Abulseoud OA, Scheidweiler KB, Huestis MA. Cannabinoid disposition in oral fluid after controlled smoked, vaporized, and oral cannabis administration. Drug Test Anal. 2017;9(6):905-915. doi.org/10.1002/dta.2092Checked Oct 4, 2026 against the abstract, and its findings as summarised in source 2.
  9. Taylor M, Lees R, Henderson G, Lingford-Hughes A, Macleod J, Sullivan J, Hickman M. Comparison of cannabinoids in hair with self-reported cannabis consumption in heavy, light and non-cannabis users. Drug Alcohol Rev. 2017;36(2):220-226. doi.org/10.1111/dar.12412Checked Oct 4, 2026 against the abstract.
  10. Wolf CE, Pokhai AA, Poklis JL, Williams GR. The cross-reactivity of cannabinoid analogs (delta-8-THC, delta-10-THC and CBD), their metabolites and chiral carboxy HHC metabolites in urine of six commercially available homogeneous immunoassays. J Anal Toxicol. 2023;47(8):732-736. doi.org/10.1093/jat/bkad059Checked Oct 4, 2026 against the abstract.
  11. United States Code, Title 7, Section 1639o(1): definition of hemp (delta-9 THC concentration of not more than 0.3 percent on a dry weight basis). www.law.cornell.edu/uscode/text/7/1639oChecked Oct 4, 2026 against the statute text.