What Causes An Invalid Lateral Flow Test

What Causes An Invalid Lateral Flow Test

Dr Mike Garside

What does an invalid lateral flow rapid test mean? 

Lateral flow testing is now far better under-stood, with most of the UK population having undertaken a lateral flow COVID-19 test over the last 12 months. Prior to 2020 few people in the UK , except those in the industry, knew what a lateral flow test was or had even heard of one, although in fact lateral flow test kits have ben widely available for many years in the form of pregnancy test kits, ovulation test kits and drug test kits.

Despite this, testing methods and procedures for lateral flow tests are multiple and varied which means there can be many causes of an invalid test result. In the diagnostic test kit industry an invalid test result is a term usually reserved for a lateral flow test kit which fails to run correctly, and which fails to give a control line (C line) at the specified read time for that particular test kit. This usually means that inadequate sample has run through the test. 

lateral flow test result invalid

 Possible causes of an absent C line on a lateral flow test (invalid test result):

  • Inadequate sample added to the lateral flow test kit
  • Sample was too thick to run the lateral flow test
  • Failing to add the buffer to the sample if there is one. Covid test kits are supplied with a buffer to dilute the sample so that it can run through the lateral flow test easily.
  • Reading the lateral flow test result too soon (always read the instructions so you know how long to wait for the result)
  • Failing to remove the cap from the lateral flow test kit. Some lateral flow test kits (for example multi-panel drug test kits) have a cap which must be removed prior to performing the test. This does not aply to covid tests which are usually cassette test kits.
  • Faulty lateral flow test kit ( this is rare)

What do I do if no C line appears on the lateral flow test kit?

Any lateral flow 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.

Are there any other causes of an invalid rapid lateral flow test result?

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

Here are a few examples:

  • 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 testing but also in finger prick or lancet capillary blood samples, especially Cholesterol testing strips and kits which require a much larger sample volume to be collected to run the 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 lateral flow 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 lateral flow test kits are concerned, they really merge.

How often do invalid results occur on lateral flow 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 testing procedure, simple timing and familiarity with the rapid testing kit that you have selected, it's not difficult to get reliable results to the manufacturers specified overall levels of accuracy every time.

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

Our UK based customer support team is here to help with any queries you have regarding rapid lateral flow testing kits we supply. You can contact them either by email or by telephone during our office hours .

Contact Access Diagnostics UK Customer Support

Available Monday to friday 8.30am to 5pm

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