Let’s learn more about POCT pregnancy tests…
At our institution, we use a POCT pregnancy test using urine. This is an immunochemical test that can additionally be used by serum to detect pregnancy. In order to obtain accurate results, those developing the test must wait THREE minutes using a urine sample and FIVE minutes using a serum sample.
The beta-HCG hormone level threshold for which this test can detect pregnancy (per the manufacturer) is 25 mIU/mL, with a specificity of 99%.
Other facts regarding the above POCT test:
What happens if you need to repeat the pregnancy test (i.e. the test itself is misplaced) – can you still use that urine sample that has been sitting in the dirty utility room?
Yes, if the sample has been at room temperature and it less than EIGHT hours old.
Does the quality of the sample affect results?
Grossly cloudy urine/urine sediment can affect the test result. On the other end of the spectrum, dilute urine (i.e. a specific gravity less than 1.005) can provide FALSE NEGATIVE results.
Can you be just “a little bit pregnant”?
It is possible that a woman can be in the very early stages of a viable pregnancy, but have a negative POCT pregnancy test. In this case (i.e. when there is a strong clinical suspicion for such), a serum, quantitative beta-HCG test can be sent. You can also have the patient repeat this POCT test in two to four days.
Also, there are different types of HCG!
There is the most common form of HCG – consisting of an alpha and beta isomer. There is also the free HCG alpha subunit, free HCG beta subunit, and the HCG beta core fragment (hCG-beta CF). All of these rise at various rates throughout pregnancy.
And one more thing….
Ever heard of the “hook effect phenomenon”?
This can happen when a human chorionic gonadotropin varient (i.e. hCG-beta CF) is present in excess (think: molar pregnancy), saturating labeled antibodies to the exclusion of other varients such as intact hCG (the hormone we see in normal pregnancy) – see below:
What about the “hook-like effect”?
Similar to the hook effect, this occurs when a human chorionic gonadotropin varient is present in excess in a assay where it is recognized by one (i.e. free floating, not fixed to the assay medium) , but not both of the antibodies, see below:
Usually the hook effect is not seen until hCG concentrations reach 1,000,000 mIU/mL (again, think: gestational trophoblastic disease). This will give you a false NEGATIVE result.
The POCT pregnancy test used at our institution has can provide a correct result (i.e. POSITIVE result), up to a beta-HCG of 500,000 hCG/mL.
References
Griffey R T, Trent C J, Bavolek R A, Keeperman J B, Sampson C, Poirier R F, “Hook-like Effect” Causes False-negative Point-of-care Urine Pregnancy Testing in Emergency Patients, The Journal of Emergency Medicine, 44(1): 155-160, 2013.
McChesney R, Wilcox A J, O’Connor J F, Weinberg C R, Baird D D, Schlatterer J P, McConnaughey D R, Birken S, Canfield R E, Intact HCG, Free HCG beta subunit and HCG beta Core Fragment: Longitudinal Patterns in Urine During Early Pregnancy, Human Reproduction, 20(4): 928-935, 2005
