What "presumptive positive" and "negative" mean

A home drug test can feel very final: a line appears or it doesn't. But the result is a screening result, and the words used to describe it are chosen carefully. Understanding them can prevent a lot of unnecessary hurt.

Screening first, confirmation second

The U.S. Food and Drug Administration (FDA) describes home drug testing as a two-step process. The home test is the quick first step. If it suggests a drug may be present, the second step is to send a sample to a laboratory, which uses a more specific chemical test. FDA calls laboratory tests the most reliable way to confirm drugs of abuse.

FDA also describes home tests as qualitative: they indicate whether a drug may be present, not how much of it.

What "presumptive positive" means

"Presumptive" means "assumed until confirmed." A presumptive (or preliminary) positive says the test reacted as if a substance it screens for might be present at or above its cutoff level. It does not, by itself, establish that the substance is there.

FDA's guidance is direct: a preliminary positive should go to a laboratory for confirmation, and you should take no serious actions until you have the laboratory's result. FDA lists several things that can affect home results, including how the test was performed, how the test or urine was stored, what the person ate or drank, and prescription or over-the-counter medicines. It specifically notes that some over-the-counter medicines can give the same result as amphetamines, and that a positive result for a prescription drug does not show it was misused.

What "negative" means

A negative means the test did not detect the substances it screens for, above its cutoffs, in that sample, at that time. That is narrower than it sounds. FDA notes that a negative doesn't prove someone hasn't used drugs, and that no test of this type is 100% accurate. Reasons FDA gives include:

• the test didn't screen for the substance in question;
• the sample was collected outside the time window in which the substance can be detected;
• the test's chemicals had degraded, for example because the kit was stored improperly or used after its expiration date.

If there's still a real concern after a negative, FDA suggests testing again later and talking with a doctor.

What a "cutoff" is

Screening tests are designed around a cutoff: a concentration, usually given in nanograms per milliliter (ng/mL), at which the test is designed to read positive. Below the cutoff, the test is designed to read negative, even if a small amount is present.

As one real example, an older FDA decision summary for a home test (510(k) number K050936) lists a designated cutoff of 50 ng/mL for marijuana (THC) and 300 ng/mL for cocaine. That's an example of how cutoffs are stated, not a standard every kit uses. Different products can use different cutoffs, so the label of the kit in front of you is what counts. Our article on reading a test kit label explains where to find them.

How the lines are read (it's often the opposite of what people expect)

Some home drug tests are built so that a line in the test area means negative, and no line means presumptive positive. The FDA decision summary mentioned above describes exactly that design for its device. Anyone who assumes "a line means positive" could easily misread a test like that.

Designs vary, so the only safe rule is to read your kit's instructions before you start. FDA notes that results must be read within a certain number of minutes after starting the test, and the instructions tell you how long that is and what a valid result looks like.

Talking about results

The National Institute on Drug Abuse (NIDA) recommends the neutral phrases "testing positive" and "testing negative" on a drug screen, rather than words like "clean" or "dirty", which it notes can reduce a person's sense of hope and self-efficacy. With a presumptive result, you might go further and say "the screen was presumptive positive; we're getting it confirmed."

A calm plan for an unexpected result

1. Check that the test was done and read exactly as the instructions say, within the stated time.
2. Check the kit's expiration date and how it was stored.
3. Note any medicines or supplements taken. A pharmacist or doctor can say whether they might matter.
4. Arrange laboratory confirmation before anyone acts on the result. See why home tests may need lab confirmation.
5. Talk about it the way your agreement says you will, ideally decided long before this moment.

P in the Cup is a scheduling and accountability service. It does not sell, administer or interpret tests, and a result someone reports in an app, or a photo of a test, is not a laboratory-verified result. The random scheduling and accountability service is free. Test kits and any laboratory testing are purchased separately.