"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


J Emerg Med. 2013;44(1):155-160. 

In This Article

Abstract and Introduction


Background: Failure to detect pregnancy in the emergency department (ED) can have important consequences. Urine human chorionic gonadotropin (uhCG) point-of-care (POC) assays are valued for rapidly detecting early pregnancy with high sensitivity. However, under certain conditions, POC uhCG tests can fail to detect pregnancy.

Objectives: In investigating a series of late first-trimester false-negative pregnancy tests in our ED, a novel and distinct causative phenomenon was recently elucidated in our institution. We discuss uhCG POC tests, review our false-negative rate, and describe mechanisms for false negatives and potential remedies.

Discussion: The false-negative POC uhCG rate is very low, but in the setting of a large volume of tests, the numbers are worth consideration. In positive uhCG POC tests, free and fixed antibodies bind hCG to form a "sandwich"; hCG is present in several variant forms that change in their concentrations at different stages of pregnancy. When in excess, intact hCG can saturate the antibodies, preventing sandwich formation (hook effect phenomenon). Some assays may include an antibody that does not recognize certain variants present in later stages of pregnancy. When this variant is in excess, it can bind one antibody avidly and the other not at all, resulting in a false-negative test (hook-like phenomenon). In both situations, dilution is key to an accurate test.

Conclusions: Manufacturers should consider that uhCG tests are routinely used at many stages of pregnancy. Characterizing uhCG variants recognized by their tests and eliminating lot-to-lot variability may help improve uhCG test performance. Clinicians need to be aware of and familiarize themselves with the limitations of the specific type of uhCG POC tests used in their practice, recognizing that under certain circumstances, false-negative tests can occur.


Failure to detect pregnancy in the emergency department (ED) can have important consequences. These may include missed ectopic pregnancy (the leading cause of first-trimester pregnancy-related maternal death), administration of medications contraindicated in pregnancy, fetal radiation exposure, patient complaints, and lawsuits. This has led to the emergency medicine dictum to check for pregnancy in all women of child-bearing age.[1–3]

Pregnancy tests in general usage include qualitative urine point-of-care (POC) tests in laboratories, clinics, and EDs; over-the-counter (OTC) urine tests for consumer/layperson use; and qualitative and quantitative serum tests. With their high level of reliability, POC tests have become a widely accepted, fast, and accurate method for determining pregnancy. Compared to serum human chorionic gonadotropin (hCG) testing, urine hCG POC tests have been reported to achieve as high as 100% sensitivity and 99.2% specificity in a clinical setting, which resulted in a positive predictive value of 98.3% and a negative predictive value of nearly 100%.[4] In addition to their accuracy, POC testing is also preferred in emergent settings due to its faster result times, with a mean time to result of 7.6 min, compared to 67.4 min for urine results from the laboratory.[5]

Like all tests, POC uhCG tests have limitations. In addition to familiar problems (incorrect reading time, insufficient/dilute urine, mislabeled specimens), false-negative results have been reported in urine-based POC qualitative hCG tests due to what is known as the "hook effect phenomenon".[6] Recently, investigation of a number of false-negative urine POC tests among patients with late first-trimester pregnancies in our ED led to the postulation of a similar but distinct "hook-like" effect that can result in false-negative urine hCG testing at later stages of early pregnancy (Table 1).[6] We describe these phenomena, present data from a review of false-negative uhCG tests in our ED, and discuss urine hCG POC testing and potential remedies for these false-negative results.