"Hook-Like Effect" Causes False-negative Point-of-Care Urine Pregnancy Testing in Emergency Patients

Richard T. Griffey, MD, MPH; Caleb J. Trent, MD; Rebecca A. Bavolek, MD; Jacob B. Keeperman, MD; Christopher Sampson, MD; Robert F. Poirier, MD

Disclosures

J Emerg Med. 2013;44(1):155-160. 

In This Article

Case Series

Among approximately 87,000 annual visits to our ED in 2009, 11,760 urine hCG tests were performed. In this time period, five false-negative urine POC pregnancy tests late in the first trimester were reported through our end-of-shift reporting system, with serum testing from the ED and subsequent confirmatory testing of mechanism in the laboratory. We present details of these cases. Patient ages ranged from 18 to 23 years, and estimated gestational ages ranged from approximately 6 to 12 weeks. Each patient asserted at triage that she was pregnant. This assertion in the setting of a negative test prompted further testing. After the POC testing results, pregnancy was confirmed with serum hCG tests and bedside ultrasound. In at least one of the cases presented, the news that the urine pregnancy test was negative resulted in much patient anxiety and grief before subsequent testing confirmed pregnancy. Embarrassment and need for explanation on the part of the provider who had initially conveyed the false-negative test results might be avoided when limitations of the uhCG testing are fully understood. Accompanying diagnoses included urinary tract infection, cervicitis, threatened abortion, abdominal and low back pain with evaluation for appendicitis, and hyperemesis gravidarum. Further details are provided in Table 1.

A subsequent 1-year record review (November 1, 2008 to November 1, 2009) was undertaken through a query of our electronic medical record to provide an estimated false-negative rate, identifying additional false-negative POC uhCG tests (Figure 1). Sixty-nine cases were identified with a negative uhCG test followed by a positive serum quantitative hCG. Chart review revealed that in 29 of these cases, notation of negative POC testing was due to documentation error that was detected in real time and noted in the chart. In the remaining 40 cases, the POC uhCG test was negative and a serum quantitative hCG was positive, giving an overall false-negative rate of 0.34% (40/11,760). In our institution, the lower threshold for POC kits is 25 IU/mL, and 5 IU/mL is the lower limit our laboratory reports for serum quantitative tests. In 18 of these 40 cases, the quantitative serum level was below the threshold for detection by POC testing, with levels ranging from 6.9 to 20.8 IU/mL. Restriction of results to negative POC results within the detection range of the test kit would yield a false-negative rate of 0.19% (22/11,760).

Figure 1.

Flow diagram for chart review. hCG = human chorionic gonadotropin; POC = point-of-care.

In nearly every case, serum testing was prompted after report by the patient of recent positive testing at home, a clinic, or in the ED. Vaginal bleeding was the most common chief complaint. Among the 22 cases with serum concentrations above the POC detection threshold, estimated gestational ages (based upon last menstrual period) ranged from 4 weeks, 2 days to 16 weeks, 1 day. Serum quantitative levels ranged from 25.4 to 268,022.5 IU/mL. It is possible that there were additional false-negative POC tests that went undetected, as POC tests are not routinely confirmed by serum hCG testing. In the majority of the cases identified, no reporting was initiated for laboratory follow-up to ascertain the etiology.

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