Is the qualitative serum pregnancy test obsolete?

I’ve written several times about qualitative hCG tests in this blog.  As a reminder, qualitative tests can be performed using urine or serum samples.  Urine tests can be performed close to the patient or even at home because the urine sample requires no special processing.  However, when serum is the test really can’t be performed at home or at the point-of-care because the blood sample has to be centrifuged to first obtain the serum and centrifugation is usually only performed in the clinical laboratory.  Notably, clinical labs are often able to do quantitative hCG testing on serum, too.

So, if a lab can do qualitative and quantitative hCG testing on serum, why not just offer one test instead of two?  In other words, might the qualitative test be considered obsolete?  My lab recently published a study that addressed that question.

To answer that question we surveyed several hundred doctors and the survey results revealed the following:

  1. When requesting serum hCG tests, 49% of physicians preferred to order a qualitative rather than a quantitative test even though they believed quantitative tests were more accurate.
  2. Physicians preferred qualitative tests because they believed that they received the test results faster.

However, when we examined the turnaround time data, that last point was not supported.  There are a few definitions of turnaround time to consider.  Doctors consider it to be the time it takes to get a result after the sample is collected while laboratorians consider it to be the time it takes to produce the result after they receive the sample.

By the lab’s definition, qualitative tests were performed more rapidly than quantitative tests but there were no differences using the doctors’ definition of turnaround time.  That’s because the time it takes to transport the sample to the laboratory is known to contribute the most to delays in the total testing process.  So, although physicians believed they get results from qualitative tests more quickly, it doesn’t seem to be the case.

We also compared the analytical sensitivities of the two types of tests.  The qualitative test that we used had a claimed detection limit of 25 IU/L.  That is, a sample with an hCG concentration above 25 IU/L should produce a positive result.  Of the samples that gave a positive result, about 20% had an hCG concentration that was <25 IU/L which indicated that the qualitative test was more analytically sensitive than we expected it to be.  In my opinion, that’s a good thing.

Because we determined the actual pregnancy status of all the patients with a positive result, we were also able to determine how well the qualitative and quantitative tests performed at determining pregnancy status.  Both tests did quite well and showed high sensitivity and specificity.  That is, there were very few false-negative or false-positive results.  From a clinical perspective, a false-negative result is more concerning than a false-positive one because a pregnant patient who is incorrectly identified as not being pregnant risks being exposed to a medical intervention that could harm the fetus.  The false-negative rate was lowest, only 0.1%, when the qualitative test was evaluated against pregnancy status and the detection threshold of 25 IU/L.  The performance of the quantitative serum hCG test was identical.  So, both the qualitative and quantitative serum hCG tests do a very good job at ruling-out a possible pregnancy.

So, given this evidence, I would conclude that while qualitative hCG tests could be replaced by quantitative tests, there is really no compelling reason to do so.

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