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How to Read Drug Test Results From the Lab

Being able to pass a drug test has long been considered a standard for job screening, and for good reason. Countless jobs demand total sobriety to be efficient. And from a business standpoint, hiring a candidate with drug use issues can lead to legal missteps, workplace injury, and damage to productivity.

Any hiring manager knows that finding a great employee, hiring them, and training them takes time. The overall goal for any hiring process is to fill a role for as long as possible. You want employees that can provide years of productivity, and you want to waste less time considering candidates that don’t. Drug testing can play a significant role in that. 

Learning how to read drug test results from the lab is essential. Here’s everything you need to know about negative drug test results, positive drug test results, and everything in between.

Types of Drug Tests

Pre-employment drug testing typically includes the following:

Urinalysis: A urinalysis test is the most common type. Drugs become detectable within a few hours of use and can be detected for up to four days after use. However, if a person routinely uses drugs, they can stay in the urine for weeks after the last use. That said, urine tests won’t measure the amount of drugs in the system. It alerts you only to the fact that the person has used them recently. 

Saliva: A saliva test is ideal for detecting drug use quickly, as drugs are detectable for up to 48 hours. Also, saliva tests often provide more info than the standard urinalysis, as results can show the amount used. Worth noting, saliva tests can detect over a dozen of the most commonly abused substances. 

Hair: Hair tests can detect drugs for months after the last use. In fact, hair that grows from the scalp can be used to detect drugs for up to 4 months. However, body hair can detect drugs up to a year after the last use, assuming the person uses drugs frequently. 

While drug tests using urinalysis, saliva, and hair are the most common, they’re not the only form of drug testing. Pre-employment drug test results can make or break a hire. But knowing how to read drug test results from the lab means being able to decipher all of the major tests. Other drug test types include:

Blood: The detection time for drug use in the blood is 48 hours. 

Sweat: The detection time for sweat drug tests is up to a few weeks. 

It’s worth mentioning that learning how to read drug test results from the lab means considering that detection times vary. Many factors can undermine a drug test, including metabolism, diet, and more. 

What a Positive Drug Test Doesn't Tell You

Positive drug test results give a lot of information. But here are a few things you won’t learn from the results.

Cut-off Levels: Cut-off levels are the minimum amount for a drug to be detected. This means that passing a drug test may not mean the person doesn’t engage in recreational drug use but instead has too low a level in their system for the test to detect. That said, the government sets cut-off levels specific to each drug. Additionally, in the case of a urinalysis, a well-hydrated candidate’s sample may be diluted, causing so few traces of a drug that it may fall below the cut-off level despite recent drug use.

Candidate History: Hiring managers should rarely, if ever, decide solely on the results of a drug test, but instead use it with other information to draw a fully formed conclusion. Why? A candidate may have positive drug test results for more than a few reasons. With the increasing legalization of marijuana, candidates may use it therapeutically to treat depression, symptoms of PTSD, etc. While certain drugs like methamphetamines may have little to no medical use on a larger scale, there are dozens of medically-relevant drugs that a candidate may be prescribed and use responsibly. Remember to ask your candidate why they tested positive before jumping to conclusions. 

Frequency of Drug Use: Go-to drug tests like urinalysis don’t tell you how frequently the drug has been used. It’s only an indicator that said the drug had been taken recently. It also won’t tell you how much of the drug has been taken. There are exceptions to this rule, as hair tests provide a reasonably in-depth picture of a candidate’s drug use over time. 

Understanding Drug Test Results

Learning how to read drug test results from the lab isn’t as complicated as it may seem. The average drug test uses a Control Line to Test Line format, meaning there will be a large C and T on the test results. A line by the T means the sample is invalid and must be retaken. A line by the C means the person has tested positive for drug use. And lastly, a line in both the C and T columns means the candidate has tested negative. The boldness of the lines is not an indicator meaning that a faint line and a bold line are considered the same with no indication as to the amount, frequency, etc.,

False positives are another factor to consider. False positives, meaning your candidate has tested positive despite not having drugs in their systems, can occur from diet, prescription medications, and other factors. Simply put, chemical compounds are complicated, and even test results can be wrong. It’s relatively rare, but it does happen. If you think a candidate is a victim of a false positive, retest them with a fresh sample. 

What Does a Negative Test Result Mean?

Negative drug test results indicate a person has no detectable amounts of drugs in their system. This is the ideal outcome of any drug test. 

What Does a Positive Test Result Mean?

Positive drug test results mean a person has drugs in their system, or the test is a false positive. More likely than not, positive drug test results indicate drug use. Just remember to dig a little deeper before making your final decision. 

What is a Cut-Off Level

The cut-off level is the minimum amount required for a drug to appear on a drug test. 

What Does ng/mL Mean on a Drug Test?

Ng/mL, short for nanograms per milliliter, is the drug concentration in the sample. Cut-off levels are based on the ng/mL for each drug. For example, the ng/mL for marijuana is 50. In comparison, cocaine’s ng/mL is 150. You can consider ng/mL levels akin to blood alcohol levels for drinking.

What Does a Pre-employment Drug Test Include?

The average pre-employment drug test only includes a urinalysis. However, hair, saliva, sweat, and blood are other options that can provide specific timeframes. The average business won’t need to utilize the entire battery, but having the option in your back pocket can bolster your resources. 


Learning how to read drug test results from the lab is an integral part of hiring. Thankfully, there’s not too much to it once you know what to look for. However, a drug test won’t give you the complete picture alone. Combine drug test results with background checks to give you a clearer understanding of your candidate. 


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