Today's post is by a guest author, Dina N. Greene, Ph.D. Dr. Greene is a Scientific Director at Northern California Kaiser Permanente Regional Laboratories in Berkeley, CA. She discovered that qualitative hCG tests may not be as analytically sensitive as we all have come to believe and she shares her observations here. A report of her work has been published in Clinica Chimica Acta.
The assessment of very early pregnancy (from conception until about two weeks following the
expected menses) is dependent on the detection of hCG in serum or urine. In health care settings a urine sample is often the specimen of choice because it is convenient and usually easy to obtain.
When urine samples are tested for hCG they are most frequently tested using qualitative (yes/no) point-of-care (POC) devices. This type of testing is attractive because it is performed close to the patient and the test results can be obtained within minutes. In general, when challenged with urine or serum containing hCG these devices work well. However, what was not known was how sensitive these devices are for detecting very early pregnancy. That is, could pregnancy be ruled out if a qualitative POC test was negative?
To answer that question we completed a study that took a systematic approach to this question by testing urine and serum specimens collected from patients that spanned a wide range of hCG concentrations with two commonly used POC devices.
While many concentrations of hCG were represented in these samples, we purposefully skewed the specimens so that a large percentage (~30%) had concentrations of hCG expected to be seen only in very early pregnancy. The results were surprising.
We found was that the devices did not always detect hCG at the lowest detectable concentration claimed by the manufacturer (20 IU/L for urine and 10 IU/L for serum). In fact, we had many false-negative results when the urine concentration of hCG was as high as 200 IU/L or the serum hCG concentration was as high as 50 IU/L. We further showed that the urine specimens were collected from patients that were at approximately 4 weeks’ of gestation which, if calculated from the day of the last menstrual period, is close to the day of expected menses.
Anecdotally, medical providers at some institutions have recognized this phenomenon. If a sexually active woman is unsure of her pregnancy status, and the POC urine hCG test result is negative, the provider may encourage the patient to return for retesting in a few days. Alternatively, if the patient’s pregnancy status must be known urgently, the provider may collect a blood sample for quantitative serum hCG testing performed in the laboratory to confirm the negative POC test result.
Interestingly, the package insert of one qualitative hCG POC device used in our study states “If a negative result is obtained, but pregnancy is suspected, another sample should be collected and tested 48-72 hours following.” Most other hCG POC devices provide a similar disclaimer. Although it is empirically recognized that false-negative results are possible in early pregnancy, most individuals (health care professionals and consumers alike) assume that this corresponds to the period of gestation that precedes hCG production. What our study showed is that hCG is present in the urine and serum of these women, but the concentration is too low for the POC devices to always detect reliably.