If you have a urine test coming up, the first thing worth knowing is that a drug test is not a yes-or-no question about whether cannabis is in your body. It is a comparison against a number. Understanding which number, and what happens on each side of it, takes a lot of the fear out of the process.
The two numbers
Federally regulated urine testing in the United States uses two different cutoffs for cannabis, and most private employers follow the same standard because their labs are built around it.
The first is the initial test cutoff of 50 ng/mL. The second is the confirmatory test cutoff of 15 ng/mL. Both are written into federal regulation: the Department of Transportation's testing rules set the marijuana metabolite initial test cutoff at 50 ng/mL and the confirmatory test cutoff at 15 ng/mL (49 CFR § 40.85), matching the HHS Mandatory Guidelines for Federal Workplace Drug Testing Programs.
Nanograms per milliliter is a concentration, not a total amount. It describes how much of one substance is in the sample you handed over, which is part of why hydration comes into the story later.
What the screen is
The initial test is an immunoassay. It uses antibodies that latch onto cannabis-related compounds and produces a fast, cheap, automated yes-or-no reading against the 50 ng/mL line. Labs run it on everything because it clears the large majority of samples in minutes.
An immunoassay is a broad net on purpose. It is sensitive but not very specific, which means it is good at noticing that something in the cannabinoid family is present and poor at telling you exactly which molecule it found. That is why an immunoassay result is a presumptive positive, never a final one.
What confirmation is
Anything that screens positive goes on to a second, separate test on a separate portion of the sample, using gas chromatography with mass spectrometry (GC/MS) or the liquid chromatography equivalent. Chromatography physically separates the compounds in the sample; mass spectrometry then identifies each one by its molecular fingerprint.
The compound being confirmed is THCA, shorthand in testing for 11-nor-9-carboxy-THC, usually written THC-COOH. It is the main inactive metabolite your liver makes after you use cannabis. It is not THC itself, and it does not get anybody high.
Confirmation is the result that counts. A screen that goes positive and then fails to confirm at 15 ng/mL is reported as negative.
Why the screen cutoff is higher than the confirmation cutoff
This trips people up constantly, because it looks backwards: the loose test uses the bigger number. The federal guidelines explain the reasoning directly. Because "immunoassays for cannabinoids react with multiple compounds that are excreted as a result of marijuana use," it "is necessary to use an immunoassay cutoff higher than that of the confirmatory test in order to detect the target analyte (THCA) at or above the confirmatory test cutoff" (HHS Mandatory Guidelines).
In plain terms: the screen is counting a whole family of related compounds at once, so it needs a higher bar to reliably flag the samples that would confirm at 15 ng/mL for the one specific metabolite. When a lab uses a more specific technology for the initial test, the rule requires 15 ng/mL for both steps (49 CFR § 40.85).
A positive is not a measure of impairment
This is the part that deserves to be said plainly, because a lot of people carry unnecessary guilt about it.
A urine test does not measure whether you are high. It measures an inactive metabolite that your body has already finished with. Even for THC in blood, which is much closer to the moment of use, the National Highway Traffic Safety Administration's report to Congress describes the correlation between concentration and impairment as poor, and says the research does not support setting a per se level the way it does for alcohol (NHTSA, DOT HS 812 440). Urine is a further step removed again.
A positive urine result also does not pin down when you used. In a monitored-abstinence study of chronic users, researchers found that "a positive specimen for THC following a negative urine specimen cannot be attributed to new cannabis use," because stored cannabinoids come back out unevenly over weeks (Lowe et al., 2009).
So the honest reading of a positive is narrow: at the moment this sample was collected, a cannabis metabolite was present above a set threshold. That is all. Policies then decide what to do with that, and policies vary enormously.
Creatinine and the dilute flag
Labs do not just measure drugs. They also measure whether the sample looks like normal human urine. That is called specimen validity testing, and the two headline numbers are creatinine and specific gravity.
Creatinine is a waste product your muscles produce at a fairly steady rate, so its concentration is a decent proxy for how watered-down a sample is. Specific gravity measures density against water.
Under federal rules, a urine specimen is reported as dilute when the creatinine concentration is at least 2 mg/dL but under 20 mg/dL and the specific gravity is above 1.0010 but below 1.0030 (49 CFR § 40.88). A specimen with creatinine under 2 mg/dL together with a specific gravity at or below 1.0010, or at or above 1.0200, is reported as substituted — meaning it does not look physiologically consistent with human urine at all.
A dilute negative is not a clean pass. Depending on the program, it commonly means an immediate retest, sometimes observed, and in some workplaces it is treated as a refusal. Drinking a large volume of water shortly before a collection is the most common way people land there by accident. If you genuinely drink a lot of water, it is worth knowing this is a real and frequent outcome.
Below the cutoff is not the same as zero
One last framing that helps. Cutoffs exist so that trace amounts do not generate endless positives, not because 14 ng/mL is meaningfully different from 16 ng/mL biologically. Crossing below the line means your concentration has fallen under an administrative threshold on that day, in that sample. It does not mean your body is finished clearing.
That distinction matters most for people who used heavily for a long time, because their concentrations drift down slowly and can wobble around the line for a while. If a test result is going to affect your job, your health, or a legal matter, talk it through with your medical review officer or your own doctor rather than guessing from a number.
What the research says
- 49 CFR § 40.85 — Cutoff concentrations for urine drug tests (U.S. Department of Transportation)
- 49 CFR § 40.88 — Criteria for dilute and substituted urine specimens
- HHS Mandatory Guidelines for Federal Workplace Drug Testing Programs, Federal Register, January 23, 2017
- Lowe RH, et al. Extended urinary Δ9-tetrahydrocannabinol excretion in chronic cannabis users precludes use as a biomarker of new drug exposure. Drug and Alcohol Dependence, 2009
- NHTSA. Marijuana-Impaired Driving: A Report to Congress, DOT HS 812 440, 2017