From the Guidelines
Blood tests for pregnancy should be considered positive when the serum human chorionic gonadotropin (hCG) value is greater than 5 mIU/mL. This threshold is based on the most recent and highest quality study available, which emphasizes the importance of referring to local laboratory guidelines due to variations in hCG assays 1. The detection of hCG in blood is a reliable indicator of pregnancy, as trophoblastic cells produce this hormone during pregnancy. It's crucial to note that values of 5 mIU/mL or less may occur in healthy nonpregnant patients, and elevated hCG levels can sometimes be unrelated to pregnancy, such as in cases of pituitary dysfunction or neoplasia.
Key points to consider when interpreting blood test results for pregnancy include:
- The sensitivity and specificity of the test, which can detect pregnancy earlier than urine tests, often 6-8 days after ovulation
- The importance of serial measurements to assess if a pregnancy is progressing normally, as hCG levels normally double every 48-72 hours in early pregnancy
- The potential for false positives due to non-pregnancy related conditions, highlighting the need for careful interpretation of results in the context of clinical presentation and medical history.
In clinical practice, blood pregnancy tests are particularly useful for monitoring early pregnancies, especially in cases of previous miscarriage, suspected ectopic pregnancy, or fertility treatments, due to their high accuracy rates above 99% 1. However, they are more expensive and require a healthcare visit, so they're not typically used for routine pregnancy confirmation unless there are specific medical concerns.
From the Research
Blood Test for Pregnancy
- A blood test for pregnancy measures the levels of human chorionic gonadotropin (hCG) in the blood 2.
- hCG is a hormone produced by the placenta during pregnancy, and its levels can be detected in the blood as early as 6-8 days after ovulation 2.
- There are different forms of hCG, including regular hCG, hyperglycosylated hCG, and free beta-subunit, each with distinct functions and detection methods 2.
Types of hCG Tests
- Most automated commercial laboratory tests, point-of-care tests, and over-the-counter tests detect only regular hCG 2.
- Some tests, such as the older style hCGb radioimmunoassay and the Siemens Immulite platform hCG test, can detect all beta-subunit variants of hCG and their degradation products 2.
- The choice of test can affect the accuracy of pregnancy detection, particularly in early pregnancy or in cases of ectopic pregnancy or gestational trophoblastic disease 2, 3.
Clinical Applications of hCG Tests
- hCG tests are essential in managing gestational trophoblastic diseases, such as hydatidiform mole, invasive mole, or choriocarcinoma 2.
- hCG tests can also be used to predict pregnancy outcomes, such as spontaneous abortion, ectopic pregnancy, or trisomy pregnancies 2, 4, 3.
- Abnormal hCG levels have been associated with adverse pregnancy outcomes, including intrauterine growth restriction, preterm birth, and low birth weight 5.