False-positive results from qualitative serum pregnancy tests

OK.  I know I've written a lot about false-positive hCG tests already, both in February and earlier this month. Bear with me as I still have more to say on this issue.

I spoke on the topic of hCG testing at a clinical lab science conference a couple of weeks ago and got this question from someone in the audience:

"I was told that one way to investigate a possible false-positive, quantitative hCG result obtained from a serum sample was to test the sample using a qualitative hCG test because interfering antibodies don't affect those tests; is that true?"

(If you are unfamiliar with the concept of interfering antibodies and want to learn more, read this.)

My response to this question was "No, that is not true."  I'm uncertain why one would think that qualitative serum hCG tests were somehow immune to any influence from interfering antibodies but that wasn't the first time I had fielded that same question.  Clearly, this is not an isolated misconception.

To support my response that qualitative tests can also produce erroneous results due to interfering antibodies I referred to a case report we described a short while ago.  You can find that report here.  The case describes a 46-year-old woman who was not sexually active yet whose blood sample produced a positive result using a qualitative serum pregnancy test.  Because the positive result was unexpected, her doctor asked us to measure the concentration of hCG in the same serum sample.  That result was less than 2 IU/L (normal is less than or equal to 5).

When we repeated the qualitative test after treating the serum sample with a blocking agent that removes interfering antibodies, the result was interpreted as negative.  A urine sample obtained from the patient also produced a negative qualitative hCG test result.  The logical conclusion was that an interfering antibody produced a false-positive result with the qualitative test.

I don't know how often this scenario might occur but it's clear that labs shouldn't rely on the notion that a qualitative hCG test is immune to any influence from interfering antibodies.

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