Challenging Drug Test Results: When Lab Evidence Is Questionable

A lab report saying you tested positive for drugs carries enormous weight in court. Prosecutors wave it like a smoking gun, juries tend to believe it, and judges rarely question it. But lab work isn’t infallible. Contamination happens, chain of custody breaks, technicians make mistakes, and equipment malfunctions. Understanding when and how to challenge drug test results can be the difference between conviction and acquittal.

Why Lab Results Aren’t as Reliable as They Appear

The criminal justice system treats lab results as near-certainties, but behind every test result is a process vulnerable to human error and procedural failures. Crime labs operate under time pressure, handle massive caseloads, and employ technicians with varying levels of training.

Lab scandals have exposed systemic problems nationwide. Crime labs have been shut down for falsifying results, contaminating samples, using faulty equipment, and cutting corners. Drug test results depend on proper sample collection, correct labeling, unbroken chain of custody, calibrated equipment, competent technicians, and accurate interpretation. A breakdown at any point compromises reliability and creates opportunities for defense challenges.

Common Problems with Drug Testing Procedures

Chain of Custody Failures

Chain of custody tracks who handled your sample from collection through testing to courtroom presentation. Every transfer must be documented with dates, times, and signatures. Gaps in this chain mean the sample could have been contaminated, switched, or tampered with.

Common Chain of Custody Problems

• Missing documentation of transfers between officers and labs
• Samples left unattended in vehicles or evidence rooms
• Inadequate sealing of sample containers
• Failure to properly identify samples at collection or testing

If a DUI lawyer in Philadelphia or elsewhere can’t prove an unbroken chain of custody, they can’t prove the test result applies to your sample. That’s grounds for exclusion of the evidence entirely.

Cross-Contamination in Labs

Crime labs test multiple samples daily, often from different cases involving different substances. Cross-contamination occurs when residue from one sample transfers to another, producing false positive results.

Equipment used to process samples can retain trace amounts of drugs between tests if not cleaned properly. Surfaces where samples are prepared can contaminate subsequent samples. Even airborne particles in the lab environment can cause contamination if proper containment procedures aren’t followed.

Equipment Calibration and Maintenance Issues

Drug testing equipment requires regular calibration to ensure accuracy. Gas chromatography-mass spectrometry (GC-MS) machines, which are standard for confirmatory drug testing, need frequent calibration checks and maintenance.

Labs that defer maintenance, skip calibration procedures, or continue using equipment with known accuracy issues produce unreliable results. When maintenance logs show irregular maintenance or calibration failures around the time of your test, the results become questionable.

Types of Challenges to Drug Test Results

Different defenses apply depending on what went wrong with the testing process.

Constitutional and Procedural Challenges

• Illegal search and seizure: If police obtained your sample unlawfully, the results are inadmissible
• Coerced sample collection: Forced blood draws without warrants violate constitutional rights
• Lack of probable cause: Testing without legal justification makes results inadmissible

Scientific Reliability Challenges

Field tests used during arrests are notoriously unreliable. They produce high rates of false positives and can react to legal substances. Field tests aren’t admissible as proof at trial, but they influence charging decisions and bail.

Even laboratory confirmation tests have error rates. No test is 100% accurate. Defense experts can testify about known error rates and explain why results should be treated with skepticism rather than certainty.

Human Error Challenges

Technicians make mistakes. They mislabel samples, record results incorrectly, misinterpret data, or fail to follow proper procedures. Obtaining lab personnel records, including training certifications, error rates, and disciplinary history, can reveal patterns of incompetence.

Understanding Different Types of Drug Tests

Test Type	Reliability	Challenge Opportunities
Field Test	Very Low	High false positive rates, reacts to legal substances
Urine Test	Moderate	Sample switching, contamination, testing window issues
Blood Test	High (if done correctly)	Chain of custody, collection procedure, storage conditions
Hair Follicle	Moderate	External contamination, cannot prove recent use
Saliva Test	Moderate	Detection window, contamination, improper collection

Each test type has specific vulnerabilities. Blood tests are considered most reliable but are vulnerable to improper collection and storage problems. Hair tests can’t distinguish between drug use and external exposure.

The Five-Step Process for Challenging Lab Results

Experienced criminal defense attorneys follow a systematic approach:

1. Obtain Complete Lab Documentation: Request full lab reports, chain of custody records, equipment calibration logs, and technician certifications.
2. Identify Procedural Violations: Review documentation for chain of custody gaps, missing signatures, time inconsistencies, or protocol deviations.
3. Consult Forensic Experts: Retain defense toxicologists who can review procedures, identify deficiencies, and provide testimony about unreliability.
4. Research Lab History: Investigate whether the lab has faced accreditation issues, audit failures, or been involved in prior scandals.
5. Develop Alternative Explanations: Present testimony showing contamination, equipment malfunction, or that the tested substance wasn’t actually an illegal drug.

This methodical investigation takes time and expertise that can make the difference in your case outcome.

Timing Issues in Drug Testing

When you were tested relative to alleged drug use matters significantly. Different drugs remain detectable for different periods.

Marijuana can be detected in urine for weeks after use, meaning a positive test doesn’t prove impairment or recent use. Cocaine leaves the system within days. Prescription medications can produce positive results for drugs you weren’t actually using.

If the timeline between alleged use and testing doesn’t align with known detection windows, that’s grounds to question whether the positive result relates to criminal conduct or stems from legal activity or testing error.

False Positives from Legal Substances

Many legal substances trigger positive results on drug tests.

Common Legal Substances That Cause False Positives

• Poppy seeds (opiates)
• CBD products (THC/marijuana)
• Cold medicines containing pseudoephedrine (methamphetamine)
• Certain antibiotics (PCP, marijuana)
• Ibuprofen in high doses (marijuana, barbiturates)

If you can establish you consumed legal products that might produce the detected substance, that creates reasonable doubt. This defense requires documentation and expert testimony explaining the cross-reactivity.

Challenging Lab Certifications and Accreditations

Crime labs must maintain certain certifications and accreditations to ensure quality control. When labs lose accreditation, fail audits, or receive citations for violations, that information is relevant to your case.

Defense attorneys can subpoena lab accreditation records, audit results, and inspection reports. Evidence that the lab had known problems around the time of your testing undermines confidence in the results. Some jurisdictions have used labs that weren’t properly accredited or employed technicians who weren’t properly certified, which can render results inadmissible.

Discovery and Obtaining Lab Records

Challenging lab results requires access to documentation that prosecutors don’t voluntarily provide. Your defense attorney must request full lab reports, chain of custody documentation, equipment calibration logs, technician certifications, lab procedures, and quality control records.

Many prosecutors resist providing this documentation, requiring court orders to compel disclosure. Without access to underlying documentation, you can’t effectively challenge the reliability of test results.

Real-World Impact of Successful Challenges

When drug test results are successfully challenged and excluded, prosecutors often have no remaining evidence. Drug possession cases frequently rely entirely on lab confirmation. Without that evidence, dismissal is likely.

Even when other evidence exists, excluding lab results dramatically weakens the prosecution’s case and creates leverage for favorable plea negotiations.

When Lab Problems Affect Multiple Cases

Occasionally, widespread problems at crime labs affect hundreds or thousands of cases. Technicians who falsified results, equipment that was systematically miscalibrated, or contamination affecting multiple samples can lead to mass dismissals.

If your case involves a lab later found to have systemic problems, you may have grounds for post-conviction relief even if you’ve already been convicted.

Frequently Asked Questions

Can I challenge drug test results if I actually used drugs?

Yes. Even if you used drugs, if the test was conducted improperly or results are unreliable, they shouldn’t be used against you. The government must prove its case properly with reliable evidence.

How much does it cost to hire an expert to challenge lab results?

Expert fees typically range from $3,000 to $10,000 or more depending on complexity. However, this investment can mean the difference between conviction and dismissal.

Can I request an independent retest of my sample?

Sometimes, but samples are often consumed during testing or degraded over time. Your drug or DUI attorney can request preservation of samples when possible. This must be done early in the case.

How long does it take to challenge lab results?

Obtaining records, consulting experts, and preparing challenges typically takes several months. This is one reason drug cases often take longer to resolve.

Protecting Your Rights

Lab results carry tremendous weight in drug cases, but they’re not infallible. Contamination, chain of custody failures, equipment problems, and technician errors all create opportunities to challenge the scientific evidence against you.

Successfully challenging drug test results requires defense attorneys who understand forensic science, know what documentation to request, have relationships with qualified experts, and can effectively cross-examine government witnesses about lab procedures.

Don’t assume that a positive drug test means automatic conviction. Question the reliability of the evidence. Demand that prosecutors prove their case with scientifically sound, properly obtained evidence.

If you’re facing drug charges where lab results are part of the prosecution’s case, reach out to us Brennan Law Offices to discuss your case and learn how experienced criminal defense can help you fight charges based on potentially unreliable lab results.