The Duquenois-Levine test is a three-chemical color test that police use at traffic stops and in the field to screen a substance for marijuana. A positive result turns a lower liquid layer purple, and that color change is enough to arrest you, but it is not proof of anything. The reagents react to compounds found in many legal plants and, critically, to legal hemp and CBD, which the test cannot distinguish from marijuana. If you have been arrested based on this test, the outcome of your case usually turns on what a laboratory finds later, not on what the officer saw in the pouch.
How the Test Works
The kit is a single-use plastic pouch containing three sealed glass ampoules. The officer places a small amount of the suspected material in the pouch and snaps the first ampoule, releasing a solution of vanillin, acetaldehyde, and ethanol. After shaking, the second ampoule is broken, releasing concentrated hydrochloric acid, which drives the reaction. The third ampoule contains chloroform, which is denser than the other liquids and sinks to form a separate layer at the bottom of the pouch.
The result is read from that bottom chloroform layer. A deep purple or violet is treated as a presumptive positive for cannabis. Any other color, or no color change, is a negative.
Why the Color Call Is Subjective
There is no instrument measuring the color. The officer makes a visual judgment, often at night, on a highway shoulder, under whatever light is available. Two officers looking at the same pouch can reasonably disagree about whether the bottom band is “deep purple” or “dark blue.” The National Institute of Justice’s own standard for these kits warns that users must be trained to understand that the reagents can produce both false positives and false negatives.1National Institute of Justice. Color Test Reagents/Kits for Preliminary Identification of Drugs of Abuse (NIJ Standard 0604.01)
What Else Turns the Test Purple
The reagents react with chemical structures found in many plants, not only cannabis. One study found that 25 of 240 non-cannabis substances produced a positive result, and a separate analysis showed 12 of 40 common plant oils and extracts triggered the same purple color the test treats as positive. Patchouli, eucalyptus, certain teas, and aromatic resins contain phenols and terpenes that can mimic the reaction. A DEA laboratory study reportedly found a 20 percent error rate in its own testing.2Bentham Open. The Non-Specificity of the Duquenois-Levine Field Test for Marijuana
The test was never designed to be definitive. It was designed as a preliminary indicator to point officers toward laboratory analysis, which is where identification actually happens.
The Hemp and CBD Blind Spot
Under federal law, hemp is cannabis with no more than 0.3 percent total tetrahydrocannabinols on a dry weight basis, and it is legal.3Office of the Law Revision Counsel. 7 USC 1639o – Definitions Marijuana exceeds that threshold and remains a controlled substance under federal law and in many states. The Duquenois-Levine test cannot tell them apart.
The reagents react to structures present in all cannabis plants regardless of THC concentration. A bag of legal CBD flower from a licensed retailer will produce the same purple color as high-THC marijuana. The test gives no quantitative measurement at all. Only laboratory analysis that measures actual THC concentration can make the legal distinction.
Since the 2018 Farm Bill legalized hemp production, people carrying legal hemp and CBD products have been arrested, had vehicles seized, and been charged based on field tests that were always going to come back positive. Some sat in jail for days or weeks before lab results confirmed the product was legal.
When Storage Undermines the Result
Even a correctly performed test can go wrong because the reagents themselves degrade. The NIJ standard requires testing at ambient temperatures between 50°F and 104°F, with relative humidity between 10 and 90 percent.1National Institute of Justice. Color Test Reagents/Kits for Preliminary Identification of Drugs of Abuse (NIJ Standard 0604.01) A kit sitting in the trunk of a patrol car through a Southwest summer or a northern winter can easily fall outside those ranges. The same standard warns that reagent and sample contamination can produce misleading results. Officers are not chemists, and the kits do not come with thermometers.
What a Positive Result Can and Cannot Do
A positive field test generally gives an officer probable cause to arrest. Under the Fourth Amendment, probable cause requires only a fair probability that evidence of a crime will be found, and courts have consistently treated a presumptive positive as meeting that bar. The arrest, the handcuffs, the impounded vehicle, and potentially a night in jail can all flow from the color change in the pouch.
What a field test cannot do is convict you. It is an investigative tool, not scientific proof. A drug possession conviction requires proving the substance’s identity beyond a reasonable doubt, and most jurisdictions require confirmatory laboratory testing to reach that standard. Gas chromatography-mass spectrometry is the accepted method; it separates a sample’s chemical components and identifies each by its molecular signature, producing both identification and concentration data the field test cannot. When a lab result contradicts the field test, the charges should be dismissed.
The Plea-Before-Lab Trap
Lab confirmation can take weeks or months, depending on the jurisdiction’s backlog. In that gap, defendants face a choice: stay in jail or take a plea deal based on the field test alone. Research from the University of Pennsylvania’s Quattrone Center found that nearly 90 percent of surveyed prosecutors allow guilty pleas without laboratory verification of the field test, and two-thirds of drug labs reported they are not even asked to review samples when a case ends in a plea.
An estimated 100,000 or more people plead guilty to drug possession each year with field test results as the evidence. The consequences of a federal possession conviction are serious. A first offense carries up to one year in jail and a minimum $1,000 fine. A second offense triggers a mandatory minimum of 15 days and a maximum of two years, with a minimum $2,500 fine. A third or subsequent conviction means at least 90 days and up to three years, with a minimum $5,000 fine.4Office of the Law Revision Counsel. 21 USC 844 – Penalties for Simple Possession State penalties vary and can be harsher.
Challenging a Field Test
The single most important step after an arrest based on a field test is refusing to plead guilty before laboratory results come back. That is simple to say and hard to do, because it requires either making bail or waiting in pretrial detention.
Defense attorneys can attack the evidence on several fronts. The most direct is demanding confirmatory lab testing. An independent GC-MS analysis typically costs between $75 and $1,000 depending on the lab and the complexity of the analysis. If the lab result is negative, the case collapses.
Beyond the substance itself, the way the test was performed is fair game. Whether the officer followed the correct sequence, whether the kit was stored within specified temperature and humidity ranges, whether the officer had adequate training on performing and interpreting the reaction, and whether lighting allowed a reliable color reading are all questions that can be raised. A motion to suppress can be filed on constitutional grounds or on the basis that procedural errors make the result so unreliable that its prejudicial effect outweighs its value. The defense carries the burden on suppression, and the motion must be filed promptly once the problem is identified; waiting until trial is generally too late.5National Institute of Justice. Law 101 – Legal Guide for the Forensic Expert – Motion to Suppress
Newer Field Tests
The gap between what the Duquenois-Levine test can do and what the law now requires has pushed development of better field tools. The 4-aminophenol test, sometimes called the Cannabis Typification test, produces different colors depending on the ratio of CBD to THC. CBD-dominant material turns pink or red, THC-dominant material turns blue, and a purple result is inconclusive.6National Institute of Justice. Chemical Identification and Optimization of the 4-Aminophenol Colorimetric Test
The 4-AP test is an improvement, but it still cannot measure the specific THC concentration in a sample and so cannot confirm whether a sample falls above or below the 0.3 percent legal threshold.3Office of the Law Revision Counsel. 7 USC 1639o – Definitions Laboratory-grade confirmation remains the only way to definitively identify a substance and measure its THC content. Until field technology closes that gap, the Duquenois-Levine test will keep producing arrests that lab results do not support.