What causes a drug test to be invalid

Posted by Access Diagnostic Tests UK Ltd on

invalid drug test result what does it mean

What does an invalid lateral flow drug test mean?

Drug testing methods and procedures are multiple and various and the term "invalid" needs a little explaining to include not just the drug test kit itself, but also the drug testing procedure used.

In its simplest definition regarding the manufacturers product, invalid result is a drug testing industry term usually reserved for a lateral flow test kit which fails to run the appropriate length of the test strip, failing to give a C or control line at the distant end of the lateral flow strip, and indicating insufficient sample may have also been delivered to the T or Test line zone.

So a strict interpretation of the results read at the indicative read time for the drug  test, any lateral flow drug test kit result which fails to show a strong clear Control or C line should be considered invalid and repeated using a fresh sample. This rule applies to all lateral flow test kits not just drug tests.

Are there any other causes of an invalid drug test result?

In practical terms there are many additional reasons to consider a drug test result to be invalid.

Here are a few examples:

  • Reading the result at the incorrect read time as specified by the manufacturer.
  • Failure to ensure the sample is fresh, not substituted or diluted
  • Failure to collect a sufficient sample to run the whole test. This is more common than you would think particularly with saliva drug testing and in particular with the larger drug test panels like the 12 panel and 13 panel saliva drug test kits which require a much larger volume of saliva and oral fluids to be collected to run all the tests than a smaller panel saliva drug test such as a four panel. Simply put the higher the number of drugs that the saliva drug test kit detects, the larger amount of oral fluid will need to be collected to run all the tests. This is one of the commonest reasons for an invalid result on a saliva drug test kit. If this happens you simply need to collect more saliva to run the remaining tests.
  • Failure to saliva swab the entire oral cavity to collect drug deposits as well as saliva. This is particularly important for Cannabis detection in oral drug tests where the test kit detects the drug residues left in the mouth cavity. 
  • Failure to leave at least 20 minutes after having anything in the mouth or smoking before collecting a saliva sample.

While the above examples stretch the definition of Invalid more towards explanations of why did we get an inaccurate test result, as far as users of the drug test kits are concerned, they really merge.

How often do invalid results occur on drug test kits?

With the quality of lateral flow test kits now so high, invalid results should only be a rarely occurring problem. With a good established drug testing procedure, simple timing and familiarity with the drug testing kit that you have selected, it's not difficult to get reliable results to the manufacturers specified overall levels of accuracy every time.

The most common reason for an invalid drug test result is poor or inadequate saliva sample collection. Invalid results are rare in urine drug testing as it is much easier to collect an adequate sample although sample tampering can occur, which can result in an inaccurate result. 

What should i do if i get an invalid drug test result?

Our UK based customer support team is here to help with any queries you have regarding drug and alcohol on-site testing. You can contact them either by email or by telephone during our office hours .

Contact UK Drug Testing Customer Support

Available Monday to friday 8.30am to 5pm

Telephone 01263 731 168 

email UK Drug Testing

We also have a wealth of information for you online in our FAQ sections and blog, where we have answered many of the common questions.


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