By Ryah Luevano
Updated: Sept 29, 2026
Published: Sept 29, 2026
Providers see it often: an in-office urine drug screen comes back positive (or negative) for something, and the lab report tells a different story. It’s not a mistake — it’s two different technologies doing two different jobs.
Presumptive Screening vs. Definitive Confirmation
An in-office or point-of-care screen is a presumptive test. It’s fast and useful for an immediate read, but it works by looking for a chemical class rather than a specific drug, which means it can react to substances that are structurally similar to what it’s actually looking for. A positive screen is a reasonable first flag, not a final answer.
How Southwest Labs Confirms Every Result
Every specimen Southwest Labs receives gets a second, more precise look. On top of a 14-drug-class screening menu, every result automatically goes through definitive confirmation testing using LC-MS/MS (liquid chromatography–tandem mass spectrometry) — a method that identifies specific compounds rather than broad classes. Southwest Labs’ confirmation menu covers 82+ analytes, giving providers a precise, defensible answer instead of a presumptive one. This two-step process protects patients as much as it protects providers: a presumptive positive that turns out to be a false alarm — from a legitimate prescription, an over-the-counter medication, or simple cross-reactivity — never has to derail a treatment plan, because confirmation catches it before a provider has to act on it.
What This Means for Your Practice
- If a screen and a lab report ever seem to disagree, treat the lab report as the more precise of the two.
- Don’t rule out a presumptive positive without confirmation — cross-reactivity is common with immunoassay screens.
- Use the confirmation result, not the screen, when a finding will affect a patient’s treatment plan.
- Reach out to your Southwest Labs clinical support team with questions about a specific result.
Southwest Labs is a locally owned, CAP- and CLIA-accredited reference laboratory based in Albuquerque, serving providers throughout New Mexico with toxicology and infectious disease testing. To review our current toxicology test menu or get started sending samples, contact our team at Info@southwestlab.com or (505) 609-LABS.