What Is Included in a Blood Drug Test for Employment?

Pre-Employment and On-The-Job Employee Bloodwork

Blood sample for testing
Photo: Rafe Swan / Getty Images

A blood drug test, which measures alcohol or drugs in the blood at the time the blood is drawn, may be used when job applicants or employees are screened for illegal drugs.

Blood tests may be required as part of pre-employment screening or may be conducted randomly by employers, especially for employees in certain occupations, or may be required after an on-the-job accident or injury. They also may be conducted periodically as a policy, or in specific incidents if the employer has reasonable suspicion that an employee is under the influence.

Blood Drug Tests vs. Urine Drug Tests

Blood tests are not used as frequently as urine drug tests, because, unlike urinalysis, a blood test does not measure drug residues that remain in the body long after the effects of the drug have worn off. (Although as we’ll discuss in a moment, the precise amount of time varies, depending on the drug, how much was used, and other factors.)

Drugs That Are Tested For

Drugs that are screened for in a typical blood test for employment purposes include amphetamines, cocaine, marijuana, methamphetamines, opiates, nicotine, and alcohol.

Why Employers Conduct Drug Tests

Employers test for drugs for a variety of reasons, including to:

  • Deter employees from abusing substances
  • Avoid problems of decreased productivity and employee absences
  • Avoid hiring people who use illegal drugs
  • Refer employees for help who have a drug problem
  • Maintain a safe work environment
  • Minimize the likelihood of legal claims due to employee impairment
  • Protect the general public
  • Comply with state laws or federal laws

When Drugs Can Be Detected in the Body

Because drugs affect each person differently, the best way to pass a drug test is to not use drugs. However, a variety of variables influence the amount of time that a drug remains detectable, including:

  • The drug's half-life (the period of time required for the amount of drug in the body to be reduced by half)
  • Your hydration
  • How often you use the drug
  • The way you ingested it (snorting, smoking, intravenously, drinking)

There is also a very short detection period with a blood test for most drugs. Many drugs stay in the for hours or days at most. However, chronic use of marijuana can lead to positive drug screens for one to two months following the last use.

MDMA and methamphetamines stay in the body for only two days. Cocaine is detectable for four. Diazepam and other drugs with a long half-life can stay in your body for a longer period of time, often several weeks or more.

False Positive Test Results

A false positive is certainly a worry for anyone taking a drug test. While this happens infrequently, a second confirmation test greatly reduces the risk to just about zero.


When you take a drug test, it's important to give a complete and accurate history of all prescription, over-the-counter, and herbal medication used.

Certain substances, as well as OTC or prescription drugs, may result in false positives due to the way in which they react with other substances. For example, poppy seeds and dextromethorphan have led to a false positive result for opiates. Decongestants (which contain ephedrine) may result in a false positive for amphetamines.

Drug Testing Legal Issues

All prospective employees for federal, state, and private employers can legally be tested for drugs and alcohol. Most current employees can be tested since state laws are generally similar to federal statutes. Employers must follow state guidelines for testing. Testing might be restricted to candidates who have already been offered a job.

Some federal employers are legally required to administer drug testing to prospective and/or current employees. For example, the Department of Transportation and the Department of Defense and their contractors must drug-test employees.

Typically, all candidates need to be treated equally, and no individual can be singled out for testing. Advance notice must be provided to candidates and employers are required to furnish information about their testing process. The employer should have a mechanism for appealing failed tests in place.

Many states, including California, require employers to verify a cause for testing currently employed workers for substances. Employers in those states must have a reasonable suspicion that the employee in question is abusing drugs and that safety or performance has been compromised. Some states can randomly test workers without a reasonable suspicion. This practice is usually restricted to situations where safety issues are a concern, but state laws vary.

Employees who take medically necessary prescription drugs are generally protected by the Americans with Disabilities Act (ADA). However, medical marijuana use is a gray area, due to the fact that marijuana is illegal on the federal level, even when it’s legalized by a state. To deal with the issue, states such as Alaska, Arizona, Delaware, and Minnesota have passed laws prohibiting an employer from firing workers who are registered medical marijuana users and test positive for the drug. Typically, employers can still fire employees who use the drug during company time or come to work under the influence.

Consult a local employment attorney or your state's attorney general if you have questions about laws in your jurisdiction.

The information contained in this article is not legal advice and is not a substitute for such advice. State and federal laws change frequently, and the information in this article may not reflect your own state’s laws or the most recent changes to the law. 

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