Urine: Not pregnant. Blood: Pregnant. How is this possible?

If you’ve taken an over-the-counter pregnancy test to find that it’s negative, it might not be the set-in stone result. Blood tests can reveal the opposite due to the different qualities it possesses to urine.

Here to explore this issue in expert detail is Mr Mahantesh Karoshi, one of London’s leading gynaecologists. Read on to find out more.

Why can blood tests reveal different pregnancy tests to urine tests?

There are several possible explanations for the difference between the results of a urine and blood test.

First, a pregnancy is usually detectable with a urine test two to three days later than in blood. This is because the urine test has a higher detection limit. Second, the urine test can give a false negative result if the urine is diluted after drinking a lot of water.

Third, hCG (human chorionic gonadotropin), exists in various isoforms. Blood contains different isoforms than urine. As a result, the urine test does not always have to give a positive result for a certain hCG concentration in blood.

How does a false negative result come about?

In some clinical cases such as molar pregnancy and choriocarcinoma, there is an entity called ‘Hook effect’.

The negative result of the urine test can possibly be explained by the 'hook' effect, in full 'high-dose hook' effect. This means that hCG is present in such an excess that the reagent in the urine becomes exhausted and the result of the test is lower than the actual hCG concentration. To what extent and at what concentration this can happen differs per test? In molar pregnancies, the hCG concentration can become so high that a high-dose hook effect is likely.

What is hCG made of?

Lastly, hCG has four major isoforms: classical hCG, hyperglycosylated hCG, free subunit HCG, and sulphated hCG. The first molecule synthesized by the embyo is Classical hCG.

1. The Classical hCG

This molecule is one the first to be secreted by the embryo. hCG is mainly secreted by the syncytiotrophoblast, less by the cytotrophoblast. Detection of hCG can be detected in the maternal blood 10 days after ovulation by clinical biology.

     2. Hyperglycosylated hCG (hCG-H)

hCG-H is useful for predicting pregnancy outcomes in women, with a first trimester suspicion of miscarriage. It isn’t considered a better tool in comparison to the classical form of hCG.

3. The Free Beta Subunit hCG (Beta HCG)

Maternal serum free -hCG is also used as a biomarker in first trimester screening for foetal Down’s syndrome.

4. The Sulphated hCG

This isoform is a secreted as the same time as LH (Luteinizing hormone) during their menstrual cycle, and is produced by the pituitary gland in non-pregnant women. Consequently, sulphated hCG’s concentration ranges around one-fiftieth of the concentration of LH. While these levels are low, sulphated hCG is exactly 50-times more potent than LH-stimulating ovulation and corpus luteum formation. It may help stimulate progesterone production during the luteal phase.

If you’d like to arrange a consultation with this leading gynaecological expert, visit Mr Karoshi’s Top Doctors profile to do so.