b-hCG Test

Human chorionic gonadotrophin (hCG) is produced by the syncytiotrophoblast, the outer cell layer of the implanting embryo. As soon as implantation begins, the hormone enters the mother's bloodstream. There it sustains the corpus luteum, so that progesterone production continues and the uterine lining is maintained.

The hormone consists of two chains. The alpha chain is essentially identical to that of LH, FSH and TSH; only the beta chain is unique to hCG. The laboratory therefore measures that beta chain specifically. Without that specificity, the result would be confounded by other pituitary hormones carrying the same alpha chain.

In serum, beta-hCG becomes detectable roughly eight to eleven days after conception. That is a few days earlier than most home tests pick the hormone up in urine. The blood test therefore has exactly two genuine advantages over a urine test: it turns positive earlier, and it gives a number instead of a line. That number is what makes it possible to compare two measurements with each other.

The ranges below give an impression of what may be measured in each week of pregnancy, counted from the first day of the last menstrual period.

Read this table with caution. The ranges overlap enormously: two entirely normal pregnancies of the same duration can differ a hundredfold, and a value falling outside the table very often still belongs to a normal pregnancy. There is no single number that counts as the normal hCG value. The table is there to help you recognise an order of magnitude, not to pass or fail your own result.

The best-known rule of thumb about hCG is that the value doubles every 48 hours in early pregnancy. That is a simplification. In a normally progressing intrauterine pregnancy below roughly 1,500 U/L, the minimum expected rise over 48 hours is about 50 per cent, and normally progressing pregnancies exist that rise more slowly than the classic doubling. As the value climbs, the rate of rise also slows. Two measurements 48 hours apart, run by the same laboratory, are therefore far more informative than one isolated result. Different laboratories use different assays and calibrations, so two numbers from two laboratories cannot simply be compared.

A second limitation matters just as much: hCG cannot tell you where a pregnancy is located. An ectopic pregnancy can produce a rising, a falling or a plateauing hCG. Location is answered by transvaginal ultrasound, not by a number. Above the so-called discriminatory zone, somewhere between roughly 1,500 and 3,500 U/L depending on the centre, a normal intrauterine pregnancy should be visible on ultrasound. Sudden abdominal pain, pain radiating to the shoulder tip, dizziness or fainting in early pregnancy is an emergency, whatever the number is doing.

A single value also cannot date a pregnancy. The weekly ranges overlap so heavily that a result without ultrasound gives no reliable gestational age.

Outside pregnancy, hCG is a tumour marker. Trophoblastic disease, such as a molar pregnancy and choriocarcinoma, produces very high values. Germ cell tumours can produce hCG in both sexes; in men the result is assessed alongside AFP and LDH. In a man, any detectable hCG is abnormal and calls for clinical assessment.

Finally, two assay artefacts explain why a surprising result is confirmed rather than acted upon. In the high-dose hook effect, an extremely high hCG saturates the assay and a falsely low value is reported. In so-called phantom hCG, heterophile antibodies react with the test: serum is positive, urine is negative, and there is no pregnancy. In perimenopausal and postmenopausal women a low hCG can also be pituitary in origin, which is a recognised benign finding and not a pregnancy.

A serum beta-hCG can become detectable from roughly eight to eleven days after conception. To keep the chance of a misleadingly negative result low, testing around or just after a missed period is the most informative. Testing earlier is possible, but a negative result at that point rules nothing out and often simply means it was too early.

To assess the trend, two measurements are usually taken about 48 hours apart. Have the second one run by the same laboratory where possible, because assays and calibrations differ and two numbers from two laboratories compare poorly.

You do not need to fast and there is no required time of day. If you take high-dose biotin supplements, mention it: biotin can interfere with some immunoassays.

The test is also used to follow whether hCG returns to undetectable after a miscarriage, a termination or a treated ectopic pregnancy, and in oncological follow-up of trophoblastic disease and germ cell tumours. If you have received an hCG injection as part of fertility treatment, wait for the interval your doctor specifies: the administered hCG itself remains measurable for days.