A preliminary test to screen pregnancy by detecting hCG in urine. Note that hCG strips are read visually and very borderline results may be subject to false positive or negative observer bias.
In general, hCG can be detected in urine (and serum) after 7 - 10 days post conception. Any positive result should be confirmed by a blood pregnancy test.
In addition to observer bias for very borderline results, false negative results can be seen in
-
Very dilute urine (e.g. early in pregnancy and or patient on an IV drip with rapid urine formation)
-
Very early pregnancy, i.e. shortly after implantation (Manufacturer claims that the test detects the presence of HCG at the sensitivity of 25 IU/L).
-
In late first or second trimester
-
Beta core fragment of hCG (bcf-hCG) is a degradation product of intact hCG and is the main form of hCG during the late first and entire second trimester. Urine pregnancy test cannot detect this form and when urine contains predominantly bcf-hCG, it can give a false negative result (variant hook effect). If the patient has missed 2 or more menstrual cycles, then serum pregnancy test (serum hCG) should be performed. Bcf-hCG is not present at significant concentrations in serum and will not interfere.
If pregnancy is still suspected, suggest a blood pregnancy test.
False positive results can be seen in other conditions that produce hCG other than pregnancy, e.g.
-
Trophoblastic disease
-
Testicular tumour, lung cancer, breast cancer (if hCG secreting)
-
Pituitary hCG (can be seen in peri/post-menopausal women or premature ovarian failure - see hCG serum entry for more information. In these cases, serum FSH is expected to be >20 IU/L.)
Note: Heterophile antibody will not cause a false elevation in urine but can in serum.
References:
1. Latifi N, Kriegel G, Herskovits AZ. Point-of-Care Urine Pregnancy Tests. JAMA. 2019;322(23):2336?2337
2. Innovacon hCG One Step Pregnancy Test Device (Urine) Package Insert