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False negative readings can occur when testing is done too early. hCG levels rise rapidly in early pregnancy and the chances of false negative test results diminish with time (increasing gestational age). [24] Less sensitive urine tests and qualitative blood tests may not detect pregnancy until three or four days after implantation. [25]
As pregnancy tests, quantitative blood tests and the most sensitive urine tests usually detect hCG between 6 and 12 days after ovulation. [25] It must be taken into account, however, that total hCG levels may vary in a very wide range within the first 4 weeks of gestation, leading to false results during this period. [26]
Pregnancy tests fall into two categories: at-home urine tests and the in-office blood test. At-home urine tests have different thresholds for the level of hCG they’re looking for, Dr. Ahmad says.
Urine-based pregnancy tests detect hCG in the urine, while blood-based pregnancy tests measure the level of hCG in the blood. [5] The presence of hCG in a woman's body indicates that a fertilized egg has implanted in the uterus and the placenta has started to form. 10 days after fertilization, significant hCG can be detected from woman's blood ...
Still, the blood values are approximately equal between the arterial and venous sides for most substances, with the exception of acid–base, blood gases and drugs (used in therapeutic drug monitoring (TDM) assays). [6] Arterial levels for drugs are generally higher than venous levels because of extraction while passing through tissues. [6]
At or after 22 weeks of gestation, fFN levels greater than or equal to 50 ng/mL are associated with an increased risk of spontaneous preterm birth. [1] A false positive fetal fibronectin result can occur if the test is performed after digital examination of the cervix or after having had intercourse.