Presumptive vs. Positive

The first point of contact in a drug investigation often involves “presumptive” field testing. These tests are designed to give Law Enforcement Officers (LEOs) an immediate indication of whether a substance is a controlled narcotic, but they lack the scientific certainty required for a courtroom conviction. In this post, we break down two common field testing methods seen in local casework.

1. Chemical Reagent Tests (NARTEC)

Manual field kits, such as those produced by NARTEC, rely on colorimetric reactions. An officer places a small sample of the suspected substance into a glass ampoule or pouch containing chemical reagents.

• The Process: If the substance reacts and changes to a specific color (e.g., blue for methamphetamine), it is marked as a “positive” field result.
• The Risk: These tests are highly subjective. Environmental factors, light conditions, and the presence of legal “look-alike” substances can lead to false positives. As seen in evidence photos, these tests are often conducted on car hoods or in other non-sterile environments.
• Inexpensive: As you can see from this NARTEC page the cost for basic kits is not high ($32).

2. Automated Digital Analysis (MobileDetect)

The MobileDetect system by DetectaChem represents a shift toward digitalizing field evidence. This system uses a specialized pouch and a smartphone app to analyze chemical reactions.

• Probable Detection: Reports generate a “Result” of “Positive Detection” for specific drugs, such as Fentanyl.
• Technical Metadata: These reports capture critical data points, including the Algorithm Version (e.g., 1.4.5) and precise GPS coordinates (e.g., ##.965981, -##.055405) where the test occurred and map.
• Human Oversight: Despite the automation, the reports still note if the result was “Visually Confirmed” by the operator, which can be a point of contention during discovery.
• Moderate price. This MobileDetect page shows 10 count “pouches” are about $35.

Why Field Tests Are Not Final

Field tests are investigative tools used to establish probable cause for an arrest, but they do not meet the forensic standards of a MSPD Crime Lab. Discrepancies between a field “Positive” and a final Lab “Certified Report” are common. In some instances, a field test may indicate Fentanyl, while more precise laboratory equipment later identifies a different substance entirely.