A preliminary positive means an initial screening test detected a substance or marker above a set threshold, but the result has not yet been verified by a second, more precise test. It is not a confirmed positive. Think of it as a flag that says “something was found” and triggers a follow-up test to make sure the finding is real. This applies across drug testing, HIV screening, and other medical diagnostics.
How Initial Screening Tests Work
Most screening tests use a method called immunoassay. It works by exposing your sample (urine, saliva, or blood) to antibodies designed to react when a target substance is present. If the substance concentration exceeds a specific cutoff level, the test registers as positive. These tests are fast, relatively cheap, and good at casting a wide net, but they are not highly specific. They can react to substances that are chemically similar to the one being tested for, which is why a preliminary positive sometimes turns out to be wrong.
In drug testing, each substance has a defined concentration threshold that triggers a preliminary positive. For cannabis, the cutoff is 50 nanograms per milliliter (ng/mL) of urine. For cocaine, it’s 300 ng/mL. Opiates also use a 300 ng/mL cutoff, while amphetamines have a higher bar at 1,000 ng/mL. If your sample falls below the cutoff, the result comes back negative. If it’s at or above the cutoff, the result is preliminary positive and moves to the next stage.
Why Preliminary Positives Can Be Wrong
False positives happen because the antibodies in screening tests sometimes cross-react with unrelated substances. Common over-the-counter and prescription medications are known culprits. Diphenhydramine (the active ingredient in Benadryl) can trigger a false positive for methadone. Certain antibiotics called quinolones, as well as the antibiotic rifampin, can cause a false positive for opiates. Even poppy seeds on a bagel contain trace amounts of opiates that can push a sample above the threshold.
The false positive rate varies widely depending on what’s being tested. For HIV antigen/antibody screening, the false positive rate per test is about 0.2%, which is very low. For hepatitis C antibody testing, it’s around 1%. Prostate cancer screening with PSA tests has a much higher false positive rate of roughly 10%, and low-dose CT scans for lung cancer can produce false positives over 20% of the time. The takeaway: a preliminary positive is not the same thing as a diagnosis, and some tests are more prone to false flags than others.
What Happens After a Preliminary Positive
A confirmatory test follows every preliminary positive. This second test uses a fundamentally different and more precise technology, typically liquid chromatography paired with mass spectrometry. Instead of relying on antibody reactions, this method physically separates and identifies the exact molecules in your sample. It can distinguish between the target substance and a look-alike medication that fooled the first test.
Turnaround time for confirmatory testing is usually about 72 additional hours after the initial screen, though this can vary by lab and testing type.
In Workplace Drug Testing
If you receive a preliminary positive on a workplace drug test, the sample automatically goes to a certified laboratory for confirmation. You generally don’t need to request this; it’s built into the process. For federally regulated testing, your sample is split into two containers at collection. Container A goes to the lab for confirmation. Container B is held in reserve, and if the confirmed result comes back positive, you have the right to request that Container B be tested at a second independent lab.
Before a positive result is reported to your employer, a Medical Review Officer (MRO) reviews it. MROs are licensed physicians trained to interpret drug test results alongside your medical history. If you have a legitimate prescription that explains the result, or if there’s another medical explanation, the MRO can verify the result as negative. You’ll typically get a chance to speak with the MRO before any final determination reaches your employer.
In Medical Screening
The same preliminary-then-confirmatory structure applies in medical diagnostics. With rapid HIV testing, for example, a preliminary positive result is disclosed to you confidentially, followed immediately by counseling and an explanation that a confirmatory test is needed. The confirmation for HIV traditionally uses a Western Blot test, and you’ll be connected with follow-up care while waiting for results. A preliminary positive HIV result does not mean you have HIV. It means additional testing is required to find out.
For other conditions like hepatitis or sexually transmitted infections, the pattern is similar: the initial screen flags a possible issue, and a more targeted lab test confirms or rules it out.
What a Preliminary Positive Means for You
If you’ve been told a result is “preliminary positive” or “presumptive positive,” the most important thing to understand is that you’re in the middle of a process, not at the end of one. No final decisions, whether about employment, treatment, or diagnosis, should be made based on a preliminary result alone. The confirmation step exists precisely because initial screens trade precision for speed and broad detection.
While you wait for confirmatory results, make a list of any medications, supplements, or foods you’ve recently consumed. This information can be valuable when speaking with an MRO or your doctor, since many everyday substances can interfere with screening tests. If you’re in a workplace testing situation, you’re not required to disclose your medications to your employer directly. That conversation happens privately with the MRO.
Most people who receive a preliminary positive and have a legitimate explanation, such as a prescription medication or a known cross-reactive substance, will see the result either overturned by the confirmatory test or cleared by the MRO review. For those whose confirmatory test does come back positive, the result is far more reliable than the initial screen and carries the weight of a verified finding.