Thursday, June 12, 2014

Variant Hook Effect

HCG pregnacny tests look at the amount of HCG in your urine. To learn more about how pregnancy looks check out our post on it.  Sometimes even though you are pregnant, you can get a false positive if you are too far along.  Pregnancy tests measure the amount of HCG in your urine and if there is too much hCG, the pregnancy test will actually say negative because it cannot process the large amount of HCG in your urine.

As pregnancy progresses, there are actually different variant forms of hCG that begin to appear in the urine. After about 5 weeks of pregnancy (i.e. 3 weeks after the expected period) concentrations of hCG beta core fragment are higher than all other forms of hCG. This is perfectly normal. Unfortunately, the concentration of hCG beta core fragment can saturate one of the antibodies used in the assay in certain pregnancy kits, and the other antibody doesn't recognize the beta core fragment. As a result, no sandwich forms and the test is read as a negative. The further into pregnancy a woman is, the more likely that this false negative will occur. Similar to the hook effect, the variant hook effect can be confirmed if testing shows a positive result after diluting the sample. 

The audio transcript form Clinical Chemistry explains more about the false negative results in their article False-Negative Results in Qualitative hCG Devices Due to Excess hCG Beta Core Fragment.   The article interviews Dr. Gronowski.  

Dr. Ann Gronowski mentions that:

 “First, it's important that clinicians are aware that these types of false-negative results can occur. I mentioned earlier that the hCG beta-core fragment is high in midterm pregnancy urine. That means urine from beyond about 5 to 8 weeks of gestation should not be used on certain devices because it has a much higher chance of producing a false-negative result”

This podcast done by Clinical Chemistry interviews Dr. Ann Gronowski.  Dr. Gronowski is an Associate Professor in the Departments of Pathology and Immunology and Obstetrics and Gynecology at the Washington University School of Medicine.

You can read the audio transcript of this podcast here

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