Drug testing plays a vital role in maintaining safety and compliance in the workplace—especially in regulated industries like transportation, healthcare, and manufacturing. But what happens when a test comes back as “Invalid”?

While most people are familiar with “positive” or “negative” results, an invalid drug test result can lead to confusion, concern, and questions about next steps.

Let’s explore what an invalid test result means, the common causes, and what employers should do when they receive one—under both DOT and Non-DOT circumstances.

What Does “Invalid Drug Test” Mean?

An invalid drug test result means the laboratory was unable to obtain a clear positive or negative result from the specimen. The issue may lie in the sample itself or may be due to technical interference during the testing process.

It’s important to note that an invalid result does not always indicate tampering.

Top Reasons for Invalid Drug Test Results

Here are the most common causes for a test being reported as invalid:

Abnormal pH Levels

• Urine pH levels are routinely monitored.
• If the pH falls outside the physiological range, the result becomes invalid.
• Causes may include:
  • Infections
  • Dehydration
  • Extended shipping time or poor storage conditions

Immunoassay Interference

This refers to substances in the urine that interfere with initial screening (immunoassay):

• May be caused by:
  • Over-the-counter or prescription medications
  • Attempts to adulterate the sample
  • Unknown chemical interactions

Low Creatinine and/or Low Specific Gravity

These tests measure the concentration of the urine sample.

• Creatinine: A natural muscle byproduct found in urine
• Specific Gravity: Compares urine density to water

If these levels are too low, the sample may be too diluted for accurate analysis.

Abnormal Physical Characteristics

Visual inspection can detect abnormalities, such as:

• Unusual color or odor
• Presence of foam
• Differences between split samples (Bottle A vs. B)

Possible Oxidant Activity

Oxidizing agents (like bleach or peroxide) may be added to alter test results.

• These may result in a test being reported as Invalid or Adulterated
• Naturally occurring substances (like menstrual blood) can sometimes mimic oxidant activity

GC/MS or LC/MS Interference

In the confirmation phase, mass spectrometry is used to verify results.

• If interference occurs at this stage, a valid confirmation can’t be made.
• This could stem from:
  • Contaminated samples
  • Unrecognized chemical interactions
  • Adulteration attempts

Invalid Drug Test Causes

Employer Guidelines

Once an invalid result is received, employers need to act consistently and in accordance with regulations or company policy.

Non-DOT Employers

• Decisions should be based on the company’s internal drug testing policy.
• Treat all employees within a job category the same, regardless of the reason for the invalid test.
• For DOT-regulated testing, the MRO is prohibited from sharing the reason a test is invalid.

DOT-Regulated Employers (Per 49 CFR Part 40)

If a test is cancelled due to an invalid result, the MRO (Medical Review Officer) will advise the employer of next steps. Here’s what employers must do:

1. Section §40.23(f) – When Retesting Under Direct Observation

• Immediately direct the employee to undergo a new collection under direct observation (either urine or oral fluid).
• Do not inform the employee ahead of time about the observed test.
• On the new test’s Custody and Control Form (CCF), include the same reason and DOT agency as the original.
• Ensure the collector conducts the observed test per regulations.

2. Section §40.23(g) – When Negative Result Is Required

If the invalid test occurred in a situation where a negative result is required (e.g., pre-employment, return-to-duty, or follow-up), the employer must:

• Direct the employee to provide another specimen immediately.

Implications: Why This Matters

• Safety-sensitive positions (e.g., truck drivers, airline staff, rail operators) must be monitored closely.
• Employers must avoid making subjective decisions based on “why” the test was invalid.
• Consistency, confidentiality, and compliance are key.

Clarity Over Confusion

Invalid drug tests may be rare, but when they occur, they can spark confusion. Understanding the causes, recognizing the procedural requirements, and following the correct protocols will help protect your company—and your employees.

Employers should update their drug testing policies to include clear instructions for handling invalid results, both DOT and Non-DOT, and train HR staff accordingly.

If you’re unsure about handling invalid or cancelled drug tests—or need to build or review your policy—InOut Labs can help. We provide drug and alcohol testing programs, policy templates, and MRO services to keep your business compliant and your workplace safe.