Urine hCG for the question, serum hCG for the trend
Expansion
Urine hCG: positive from about 1 to 2 weeks after conception, and detects concentrations around 20 to 25 IU/l. Sensitive, cheap and adequate for the yes or no question.
Serum hCG: quantitative, and used for the trend.
- In a normal early intrauterine pregnancy, hCG roughly doubles every 48 hours
- A suboptimal rise, a plateau or a slow fall suggests an ectopic or a failing pregnancy
- A fall of more than 50 per cent in 48 hours suggests a complete miscarriage
- Very high levels, beyond those expected for dates, suggest multiple pregnancy or molar pregnancy
Discriminatory zone: above an hCG of about 1500 IU/l, an intrauterine pregnancy should be visible on transvaginal ultrasound. An empty uterus above this level raises strong suspicion of an ectopic pregnancy, though a multiple pregnancy can shift the threshold.
Pregnancy of unknown location describes a positive test with no pregnancy visible on scan, and is managed by serial hCG and repeat scanning, since it may be an early intrauterine pregnancy, a failing pregnancy or an ectopic.
False positives: trophoblastic disease, some germ cell tumours, heterophile antibodies, and recent hCG injections used in fertility treatment.
False negatives: testing too early, very dilute urine, and the hook effect, where an extremely high hCG saturates the assay and produces a falsely negative or low result, classically in molar pregnancy. Dilution of the sample resolves it.
hCG persists for several weeks after miscarriage or termination, so a positive test does not necessarily indicate an ongoing pregnancy.