Urine pregnancy test (hCG)

Principle

• Human chorionic gonadotropin (hCG) is made by the trophoblast shortly after implantation.
• Most tests are lateral flow sandwich immunoassays: hCG binds a labeled antibody (often colloidal gold) and is captured by a second antibody fixed at the test line.
• Antibodies are usually directed at the β-subunit, which gives specificity over LH, FSH and TSH (which share the α-subunit).

Specimen

• Random urine; first morning urine is preferred early in pregnancy because it is most concentrated.

Reagents and equipment

• Cassette, strip or midstream device at room temperature; timer; positive and negative control materials.

Procedure

• Check expiry and bring the device and specimen to room temperature.
• Add the stated number of drops (or immerse the strip to the maximum line).
• Read at the exact time specified by the manufacturer (commonly 3–5 minutes).

Results

• Positive: control line plus a test line of any intensity.
• Negative: control line only.
• Invalid: no control line; repeat with a new device.
• Typical detection limit 20–25 mIU/mL (some tests lower); hCG is usually detectable in urine around the time of the first missed period.

Quality control and pitfalls

• Run external positive and negative controls with each new lot or shipment and per local policy.
• Readings after the stated time may show faint evaporation lines and must not be used.
• False negatives: testing too early, dilute urine, and the hook effect with very high hCG (e.g. molar pregnancy), which can be excluded by diluting the sample.
• False positives: recent pregnancy or abortion (hCG persists for weeks), hCG injections, hCG-secreting tumors (gestational trophoblastic disease, germ cell tumors) and pituitary hCG in perimenopause.
• Ectopic pregnancy may give a weak or negative result; use quantitative serum β-hCG with ultrasound.

Clinical use

• Confirmation of pregnancy, and pregnancy screening before radiology, surgery or teratogenic drugs.