Biology Question #8861

Charlie, a 21 year old male from Astana, kazakhstan asks on December 14, 2012,

How is it that the over-the-counter pregnancy strip test is so fast and specific versus the common lab ELISA test which takes much longer incubation times and washings?

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The answer

Wendy Hutchins answered on December 14, 2012

The biggest issue with antibody tests is not the specificity of the antibody with its target, but the way that you remove any un-reacted reagents from the platform of your test.  In the ELISA, yes, you add something to the well, then wash the excess off, then add something, incubate, wash the excess off.  And keep going until you have just the substrate to apply.  Every step takes time.

In the stick pregnancy tests, the antibodies are immobilized onto an absorbent surface that is related to disposable diapers!  I’ve modified this information below from the Randox website:

The test strip contains latex colloidal particles coated with specific monoclonal (mAb) antibodies to hCG, which bind any hCG present in the urine.  You put the sample on at one end and the liquid migrates up, reacting with these mAbs.  You can’t see them before doing the test as they are spread out throughout the paper. The sample migrates along the test strip and antibody bound hCG complexes are immobilized on the test line.  The line is where another antibody to hCG is present, thus making this a sandwich type assay.  This appears as a pink-red line on the strip, indicating a positive test (or whatever colour they make the latex).  Any unbound colored particles migrate further along the strip until they are immobilized on the control line, forming a blue line.  This antibody is an anti-mAb antibody; ie: anti-mouse, immobilized in the control line.   The control line indicates that the test has been performed properly and will be visible for both positive and negative samples. Advantage, no washing, incubating or colour development.  Dipstick antibody tests have been made to a number of other important tests as well. 

A good animation of this can be found on the Kuby Immunology textbook website.

In some cases, the dipstick assay is as sensitive as an ELISA for hCG urine testing.  For serum testing, the dipstick won’t work as the serum interferes with the flow of the sample through the strip material.  In that case, yes, an ELISA works where the dipstick won’t.  There are some other “platforms” for hCG testing that still use antibodies but differ in ways of finding the binding from ELISA.   These, usually automated, assays provide much greater sensitivity for hCG so that pregnancy can be detected in suspected cases of ectopic pregnancy (fertilized egg implants in the fallopian tube days before the first missed period).

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