Detecting hCG in urine: how low is low enough?

A recent post on this blog described the inability of qualitative point-of-care (POC) hCG tests to detect hCG when it was present in urine or serum at a concentration that should, according to the test manufacturer, always be detected. The inability of these devices to detect hCG is a serious concern.

A false-negative result from a home pregnancy test can be initially disappointing if a pregnancy is desired or a temporary relief if it is not. By contrast, a false-negative result in the health care setting can result in serious harm to the fetus if a patient who is assumed to not be preganant undergo interventions that are potentially harmul to the pregnancy.

A recent case report has been published that, like other reports, emphasizes the limitations of qualitative urine hCG testing. The case describes a young woman who required radioactive iodine therapy for Grave's disease. Importantly, this young woman was also recently pregnant. That fact would likely not have been discovered had the physician relied on a qualitative urine hCG test. Fortunately, the laboratory had performed a quantiative urine hCG test (note that quantitative hCG tests using urine may be performed by the lab but the results are reported as qualitative (e.g. yes or no) and not the actual hCG concentration) which was interpreted as "positive" because the measured hCG concentration was greater than the lab's cutoff of less than or equal to 5 IU/L (it was 12 IU/L). A serum hCG test performed the same day produced a result 15 IU/L (not pregnant=less than or equal to 5 IU/L). Two days later a repeat serum hCG test produced a result of 147 IU/L confirming that she was in the very early stages of pregnancy. 

As has been noted in this blog in the past (here and here), urine hCG testing is commonly performed in the health care setting because it is convenient. However, the problems with urine hCG tests are so numerous (see here and here) that urine hCG testing should not be relied upon to determine a patient's pregnancy status.

The authors of the case described above correctly point out that the detection thresholds of most qualitative urine hCG tests are stated to be 20–50 IU/L (recent evidence suggest these cutoffs are not always accurate). Further, they call for more sensitive qualitative urine hCG tests in order to decrease the number of false-negative hCG results in the health care setting and suggest that a detection threshold of 5 IU/L (which is the same threshold used for interpreting quantitative serum hCG tests) should be used. Interestingly, this conclusion is similar to the one my group suggested in regards to qualitative serum hCG testing.

I am in complete agreement that when it comes to the detection of early pregnancy, hCG tests that are capable of accurately detecting and/or measuring hCG are required. Currently, this means that serum hCG tests should be used exclusively, in the health care setting, for this purpose. To rely on less sensitive tests and less accurate urine hCG tests is a disservice to our patients.

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