Testing urine samples for the presence or absence of hCG is commonly performed in hospitals and clinics for the rapid assessment of a woman's pregnancy status. This topic has been discussed several times in this blog (see here and here).
However, urine is not a suitable sample type for pregnancy assessment for many reasons, such as:
- Urine hCG tests are prone to false-negative results.
- hCG becomes detectable in the urine after it appears in the blood, so urine tests are not as sensitive as blood hCG tests.
- Urine tests provide a positive/negative results whereas blood tests provide a quantitative concentration.
Because of these limitations, in particular the risk of false negative results, I've often said that urine hCG testing is inappropriate in healthcare delivery settings. Blood tests for hCG are much more reliable but they take longer to produce results because of the time required for sample transport and processing by a centralized laboratory.
At long last, a rapid, quantitative blood test for hCG is finally available in the US from Abbott Point of Care, Inc. on their i-STAT instrument. Naturally, people will want to know how the test performs and we recently published a paper on the analytical performance of the test.
We evaluated the test using whole blood and plasma (the sample types that were cleared for use) as well as serum. Overall, the test works quite well in all sample types and is suitable for use in healthcare settings. It provides the reliability of laboratory blood-based hCG testing but with the convenience of point-of-care testing.
There are two important limitations to note:
- The analytical measuring range of the i-STAT hCG test is limited to 5-2,000 IU/L. While this is similar to the measuring ranges of many laboratory hCG tests, the instruments used in labs can automatically dilute and re-test samples that have a high hCG concentration, something that the i-STAT is unable to do. When tested with the i-STAT, a sample with an hCG concentration greater than 2,000 IU/L will be reported as such. While this type of result indicates that hCG is present in the blood, not having an absolute concentration will be insufficiently informative in some clinical situations.
- The test is susceptible to the high-dose hook effect at an hCG concentration somewhere between 400,000 and 600,000 IU/L. This means that when a sample with an extremely high hCG concentration is tested, the result can be falsely decreased. However, even though the result is falsely decreased, it is still very likely to be interpreted as "positive" and the risk of a false-negative result is extremely remote.
This type of test is long overdue in the US. The use of urine hCG testing to determine a woman's pregnancy status is fraught with difficulties and is known to cause harm to patients. Despite their problems, urine hCG testing won't be going away any time soon but the availability of a test that performs hCG blood tests close to the patient is a step in the right direction.