In previous blog posts we have discussed false negative urine hCG tests due to high concentrations of hCG beta core fragment (hCGβcf), the predominant form of hCG found in urine after six weeks of pregnancy. High concentrations of hCGβcf saturate either one or both of the antibodies used in the test. This hCG variant effect prevents the development of a positive signal and generates a negative result despite the presence of intact hCG in the urine sample being tested.
The original studies were performed by adding increasing concentrations of purified hCGβcf to a urine sample obtained from a pregnant woman. The problem with this approach is that the relative abundance of hCG, hCGβcf and other hCG variants in the urine that may affect device performance vary between women. A standardized method, using defined concentrations of hCG, is needed to evaluate the performance of currently available devices.
In a recent publication, we describe a screening method that can be easily used to examine the effect of hCGβcf and have used this method to test eleven POC hospital urine hCG devices.
First, a wide range of purified hCG and hCGβcf concentrations were combined to prepare 2 solutions:
- Solution A: 500 pmol/L (171 IU/L) intact hCG with 0 pmol/L hCGβcf and
- Solution B: 500 pmol/L intact hCG with 500,000 pmol/L hCGβcf
These solutions were tested on two hCG devices, the results of which helped to define a screening method:
Each device is tested with the two solutions and the intensity of the test bands are compared. If Solution B shows a lighter test band than solution A, then the device is susceptible to false negatives with hCGβcf.
Using these two solutions we compared the performance of 11 hospital urine hCG devices
- SP hCG Combo Rapid Test, Cardinal Health
- OSOM hCG Combo Test, Genzyme Diagnostics
- hCG Combo, Alere
- ICON 20 hCG, Beckman Coulter
- ICON 25 hCG, Beckman Coulter
- Elite Plus hCG, Cen-Med
- Clinitest hCG Pregnancy Test, Siemens
- hCG Urine Test, McKesson
- QuickVue+ One-Step hCG Combo Test, Quidel Corporation
- QuPID One-Step Pregnancy Test, Stanbio Laboratory
- Sure-Vue Serum/Urine hCG-Stat, Fisher HealthCare
Interestingly, we found that only 2 devices were acceptable (i.e. not affected by the hCG variant effect). These were the Beckman Icon 20 (shown above) and the Alere hCG Combo devices. By contrast, the Genzyme OSOM and Cen-Med Elite Plus hCG devices were the most susceptible to false negative results due to hCGβcf (OSOM shown below). The remaining seven were moderately affected.
The paper also demonstrated that devices that gave the strongest signal with hCGβcf alone were those that were least likely to show a false negative effect.
The screening method can be used by device users and manufacturers to evaluate hCG devices for inhibition by hCGβcf. We hope that the results of this study will help healthcare providers make informed decisions about which hCG devices to select, especially in medical centers that are unable to perform rapid, quantitative measurements of hCG in serum.
However, while the availability of hCG test devices that are not affected by hCGβcf is certainly reassuring, quantitative serum measurement of hCG should still be the test of choice when available.