Many people are surprised at how many different types of "pregnancy tests" are available. The use of quotations is deliberate because pregnancy tests don't detect pregnancy, they detect a hormone produced during pregnancy: human chorionic gonadotropin (hCG). But, because hCG is usually only present during pregnancy, one can forgive the non-specific label.
Here's a table that illustrates the available hCG tests:
So hCG tests can be performed on urine or blood using a qualitative (yes/no) or a quantitative (concentration) test. Because there is no clinical need to quantify hCG in urine, quantitative tests are usually not performed in urine. Some laboratorians (yours truly included) do believe that quantitative urine hCG tests can be helpful in specific situations but that's a post for a different day.
In this post, I want to focus on the qualitative hCG test performed with a blood sample. A recent paper suggested that this test should be considered antiquated (disclaimer: I was an author on that paper). There are a few reasons why this might be true:
- The biggest value of qualitative hCG tests is that they can be performed at the point-of-care with test results available to the physician very quickly. However, blood samples have to be centrifuged to separate the liquid part of the blood (serum) from the solid part (blood cells), and the centrifugation step is nearly always performed in the laboratory. As such, qualitative hCG tests using a blood sample cannot be performed at the point-of-care.
- The vast majority of clinical laboratories can perform quantitative serum hCG tests. Although these tests may take a bit longer to perform than qualitative tests, the time it takes to perform them is still rather fast (~20 minutes).
- Quantitative hCG tests can probably measure hCG at a concentration lower than the detection thresholds claimed by qualitative tests (~1 IU/L vs. 10-25 IU/L, respectively). If that's the case, why not use the most sensitive hCG test for a blood sample?
- The only clinical use of a qualitative serum hCG test is to detect (or rule out) a possible pregnancy. Therefore, it can be argued that the most analytically sensitive test available (i.e a quantitative test) should be utilized.
Given those reasons, I've been wondering if qualitative blood tests for hCG could be abandoned. Why bother using them if quantitative tests are readily available and can detect hCG at lower concentrations than qualitative tests? Seems like a no-brainer to me but, as a scientist, I need data to support my hypothesis. This is something to investigate!