Drug testing plays a critical role in workplace safety, compliance, and accountability. While most test results come back either negative (no drugs detected) or positive (drug or metabolite detected), there are cases where the laboratory cannot issue a definitive result. In these cases, the outcome is reported as Invalid.

An invalid drug test result does not confirm or rule out drug use. Instead, it indicates that something prevented the laboratory from completing the test successfully. Causes range from medical or biological conditions to specimen handling errors or even attempts at tampering. The interpretation and next steps depend on whether the test is DOT-regulated (covered under 49 CFR Part 40) or Non-DOT (company policy applies).

Why a Drug Test May Be Reported as Invalid

The laboratory evaluates specimens through several checks before reporting a result. If a sample does not meet testing requirements, it is flagged as invalid. Below are the most common reasons:

1. Abnormal pH Levels

The normal urine pH range for human urine is approximately 4.5 to 9.0. If a sample falls outside this range, the result cannot be validated.

Possible causes:

  • Urinary tract infections or kidney conditions
  • Certain medications
  • Extended shipping or exposure to high or low temperatures
  • Occasionally, deliberate tampering

While unusual pH levels sometimes raise concerns about adulteration, they are often explained by legitimate health or handling factors.

2. Immunoassay Interference

The immunoassay is the initial screening method that uses antibodies to detect drugs. Interference prevents accurate detection.

Possible causes:

  • Prescription drugs (e.g., antibiotics, antidepressants)
  • Over-the-counter medications (e.g., cold remedies)
  • Presence of unknown chemical compounds
  • Substances introduced to intentionally disrupt testing

When interference occurs, the screening cannot provide reliable information, leading to an invalid result.

3. Low Creatinine and/or Low Specific Gravity

Laboratories measure creatinine (a byproduct of muscle activity) and specific gravity (urine concentration). These values ensure the specimen is suitable for testing.

Possible causes:

  • Excessive fluid intake before the test
  • Natural variation in some individuals with lower muscle mass
  • Substitution of the specimen with water or another liquid

If both creatinine and specific gravity are out of range, the specimen may be reported as invalid, dilute, or substituted, depending on the values.

4. Abnormal Physical Characteristics

Specimens with unusual physical traits raise concerns about their integrity.

Examples include:

  • Unexpected color (e.g., bright blue, green)
  • Unusual odor suggesting chemical additives
  • Foamy or inconsistent appearance
  • Discrepancies between split specimens (Bottle A and B)

5. Possible Oxidant Activity

Oxidizing agents can chemically destroy drug metabolites, making them undetectable.

Possible causes:

  • Bleach, hydrogen peroxide, nitrites, or iodine added to the specimen
  • Presence of blood in urine due to menstruation, infections, or tumors

Depending on the oxidant concentration, results may be reported as Invalid or, in clear cases of tampering, Adulterated.

6. GC/MS, LC/MS, or MS Interference

Confirmation testing through Gas Chromatography/Mass Spectrometry (GC/MS) or Liquid Chromatography/Mass Spectrometry (LC/MS) precisely identifies drug metabolites. Interference at this stage prevents confirmation.

Possible causes:

  • Foreign chemicals in the specimen
  • Rare unknown factors in the urine composition
  • Intentional adulteration attempts

Since confirmation testing is essential for accuracy, any unresolved interference results in an invalid outcome.

7. Abnormal Total Protein in Oral Fluid Testing

Albumin, a naturally occurring protein in saliva, confirms that the sample is human and suitable for testing. Abnormal levels suggest contamination or unsuitability.

Possible causes:

  • Medical issues affecting protein levels
  • Contaminants from food, drink, or foreign substances
  • Attempts to interfere with oral fluid integrity

Invalid Test Causes

Reason What It Means Possible Causes
Abnormal pH pH outside 4.5–9.0 range Infection, meds, shipping issues, tampering
Immunoassay Interference Antibodies fail to detect accurately Medications, chemical interference
Low Creatinine/Specific Gravity Sample too dilute or substituted Excess fluids, low muscle mass, substitution
Abnormal Physical Traits Specimen visually inconsistent Odd color, odor, foam, split sample mismatch
Oxidant Activity Oxidizers disrupt drug detection Bleach, peroxide, nitrites, blood in urine
GC/MS or LC/MS Interference Confirmation process disrupted Adulteration, unknown interference
Abnormal Protein (Oral Fluid) Protein levels inconsistent with human saliva Medical condition, contamination

What Happens After an Invalid Result?

  • DOT-regulated tests: A Medical Review Officer (MRO) follows official procedures outlined in DOT Part 40.
  • Non-DOT tests: Employers rely on company policy, which may require retesting or further investigation.

An invalid result does not equal a failed test. Instead, it signals that the lab could not complete the process.

Additional Resources

Contact InOut Labs

If you have questions about invalid results or want to strengthen your workplace drug testing program, contact us today:

InOut Labs
Phone: (847) 657-7900
Email: info@inoutlabs.com