False negative pregnancy tests in the emergency department (ED) can have serious consequences—including birth defects or loss of pregnancy—if pregnant women are subjected to certain treatments potentially harmful to a fetus. While in vitro device manufacturers and ED clinicians must do their part to help minimize the frequency of false-negative test results, laboratory personnel ultimately are responsible for the accuracy of point-of-care (POC) testing. To minimize false-negative qualitative human chorionic gonadotropin (hCG) test results, laboratorians must understand the limitations of POC hCG devices, utilize strategies to investigate results that are inconsistent with the clinical presentation, and recommend alternate testing to help establish a definitive diagnosis.
Measurement of hCG represents an analytical challenge, as the range of hCG concentrations associated with normal pregnancy spans from 0 IU/L immediately following conception to approximately 200,000 IU/L by weeks 8 to 10. False-negative urine hCG results could be encountered for a number of different reasons. Negative results are common in very early pregnancy, when hCG concentrations in urine likely are below a device's limit of detection.
False-negative results may also occur due to the hook effect, a phenomenon characterized by a pathologically high concentration of intact hCG that saturates all available binding sites and prevents an antibody-hCG-antibody sandwich from forming. In normal pregnancy, intact hCG concentrations are not sufficiently elevated to cause a hook effect. However, intact hCG is not the only variant observed in normal pregnancy, and hCG POC devices may either recognize or interfere with these other variants. One such variant, hCG β core fragment (hCGβcf), is present at 10-fold higher concentrations than intact hCG in urine beginning at around week 6 of pregnancy. As a degradation product formed during renal filtration, hCGβcf is exclusively found in urine. Of particular interest to laboratory personnel, false-negative POC hCG results have been documented in women with high urinary concentrations of hCGβcf.
We recently reported on a screening study in which we compared the performance of commonly used POC hCG devices to evaluate their susceptibility to hCGβcf-related false-negative results (Clin Chem 2014; 60:667–74). We were surprised to find that hCGβcf concentrations observed in normal pregnancy negatively affected nine of the 11 devices we evaluated.
Given the limitations of currently available POC hCG devices, we strongly encourage manufacturers to improve their devices so that they will no longer be susceptible to inhibition by hCGβcf. The prevalence of false-negative results due to hCGβcf is unknown, but anecdotal evidence and frequent questions from clinicians regarding negative POC hCG tests in patients with a confirmed pregnancy indicate they may be an all too common occurrence. While selected manufacturers recently have improved their devices to minimize hCGβcf interference, the majority of devices remain susceptible.
We also encourage ED clinicians to re-evaluate how they use hCG POC testing. Often, hCG POC testing is performed inappropriately on all women who present to the ED, regardless of their symptoms. Indiscriminately using this test can lead to confusion if it produces a negative result for a woman known to be pregnant. Results inconsistent with the patient's presentation usually delay treatment, thereby defeating the purpose of performing rapid POC testing. Therefore, we encourage clinicians to evaluate patients' pregnancy status through quantitative hCG serum measurement in the central laboratory instead of with POC qualitative urine testing. Serum quantitative assays are more sensitive, with lower limits of detection of 5 IU/L compared to 20 or 25 IU/L seen with qualitative devices. Serum quantitative assays also are not susceptible to inhibition by hCGβcf, and are rapidly available at most major institutions.
If serum quantitative measurement is unavailable, additional strategies can help identify false-negative results caused by hCGβcf and even help the laboratory avoid these false-negative results altogether. When laboratorians confront a negative hCG POC result caused by elevated concentrations of hCGβcf, diluting the urine sample should generate a positive result. We also recommend that each laboratory perform a side-by-side evaluation of multiple devices and select the device that offers the best combination of sensitivity and resistance to inhibition by hCGβcf.
Although currently available POC hCG devices present a number of opportunities for false-negative results, it is our expectation that the frequency of these false-negatives will be reduced dramatically through the coordinated efforts of manufacturers, clinicians, and laboratory personnel.
Ann M. Gronowski, PhD, DABCC, is a professor of pathology, immunology, and obstetrics and gynecology at Washington University School of Medicine in St. Louis.
Robert D. Nerenz, PhD, is a clinical chemistry fellow at Washington University School of Medicine in St. Louis.