Immunoassay Drug Test

10 Aug

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Below is a guide to immunoassay testing for drugs that explains the basic principles the test is based on and the different methods or variations of the test as its practiced.


What is an Immunoassay Drug Test?

It’s a drug test based on identifying the antibodies that bind to specific drugs or drug metabolites in a urine sample.

Popular immunoassays are inexpensive and usually used to pre-screen a sample before subjecting it to more expensive (and reliable) tests.

Basic Principles of Immunoassay Drug Testing

An immunoassay is a test that can identify specific substances in a sample of multiple substances. (Source)

For example, in a urine sample.

Identification is based on the following principles:

  • Antibodies, called an antigen, will bind to specific a molecule or group of molecules.
  • Identification of drugs based on the ability to identify antibodies that bind to specific molecules – either drugs or drug metabolites – in the sample.

There are different ways to identify the antibody/antigen and thus test for drugs in a sample.

Next we’ll look at those different methods used to make this identification.



Immunoassay Drug Test Methods

In principle there are many different methods available to make the identification within a sample of specific antibodies that indicate the presence of a drug. In practice, there are a handful that are used, with the first one below, EMIT, being the most widespread.

  • EMIT Drug Test
    Enzyme Multiplied Immunoassay Technique

    This is the most popular and widespread test used by employers and at home drug testing kits.
    If you have to take a urine test for a job, most likely it will be for an EMIT test. In the case of marijuana, the cut off level is usually 50 ng/ml. Note that if results indicate the presence of drugs in a sample, the testing rarely steps here: the EMIT is merely used to pre-screen a wide amount of samples for the cheapest price. Those samples that test positive are then sent in for further testing using a more expensive yet more reliable technique.The EMIT uses an enzyme as its method of detection.
  • RIA Drug Test
    Radio immunoassay

    In an RIA test, a radioactive isotope is used to make the identification. These tests are rarely used outside of the military. They are much more sensitive and reliable than the EMIT method.
  • FPI Drug Test
    Fluorescence Polarization Immunoassay

    In an FPI test, flourescent compounds are used to identify the binded antibodies. Like the RIA, FPI tests are very sensitive yet not widespread.

Immunoassay Drug Tests and False Positives

For the EMIT test, false positives can run high. In particular, over the counter cold medication can result in a false positive for amphetamines on an EMIT. However, usually if a sample tests positive in an EMIT test, the results are double checked using a much more stringent and reliable testing method, typically gas chromatography / mass spectrometry, or known more simply as a GC MS drug test.

For parents performing at home testing, note that the instant “dip stick” urine cup kits use an immunoassay. For a more reliable method, a hair follicle test or a swab drug test should be considered.

For RIA and FPI immunoassay testing, false positives are rare.

Cheating and Passing an Immunoassay Drug Test

Methods used to adulterate a urine sample (including submitting fake urine in order to pass an EMIT drug test are widespread. Please review GNC Products to Pass a Drug Test to get an idea of what these products look like and how they work in regards to an immunoassay drug test.


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