Pregnancy tests can be determined at both the blood and urine levels.
Both tests can diagnose pregnancy by testing for a chemical called hCG, but with different sensitivity and specificity. This hormone has a specific value depending on the days or weeks of pregnancy and generally doubles in value every two days.
Blood tests measure the quantitative levels of the hormone beta-hCG (human gonocorionic hormone) that appears in the early stages of embryo implantation. The result is sensitive at blood level already with the presence of 1 to 5 mIU/ml of hCG, while at urine level it must be higher than 50 mIU/ml of HCG, which delays its positivity in the latter by a few days.
One thing to note about these tests is that the blood test will give us an absolute value, i.e. a clear number, which at a medical level and depending on the situation of each woman, we can not only diagnose pregnancy but also serious pathologies of the trophoblast.
In recent years, the pharmaceutical industry has improved the sensitivity of urine tests or home tests. Nowadays, urine flow measurement tests are available on the market in both strips and digital formats that can detect hCG levels above 10-15 mUL/ML. It is important when performing these tests to follow the correct indications, as there can be many failures or false negatives due to poor collection technique or the concomitant use of certain medicines that are also eliminated via the kidneys.
Approximately 10 days after an embryo transfer at the blastocyst stage and 15 days after an artificial insemination, hCG levels should be measured in the blood, if implantation has occurred, while urine tests should wait a few more days depending on the type of home test.