At what point do urine versus serum beta-human chorionic gonadotropin (BHCG) tests become detectable?

From the Guidelines

Urine beta-human chorionic gonadotropin (β-hCG) testing can detect pregnancy around 12-14 days after conception, while serum β-hCG testing can detect pregnancy earlier, around 8-10 days after conception. The difference in detection time is due to the varying sensitivity of the tests, with serum tests able to detect lower concentrations of β-hCG (as low as 5-10 mIU/mL) compared to urine tests (which typically require 20-50 mIU/mL) 1. According to the U.S. selected practice recommendations for contraceptive use, 2013, most qualitative pregnancy tests approved by the U.S. Food and Drug Administration (FDA) report a sensitivity of 20-25 mIU/mL in urine 1.

Key Points to Consider

• The sensitivity of a pregnancy test is defined as the concentration of human chorionic gonadotropin (hCG) at which 95% of tests are positive 1.
• Pregnancy detection rates can vary widely because of differences in test sensitivity and the timing of testing relative to missed menses 1.
• Some studies have shown that an additional 11 days past the day of expected menses are needed to detect 100% of pregnancies using qualitative tests 1.
• For the most accurate results, morning urine is preferred for home pregnancy tests due to higher concentration of the hormone.

Clinical Implications

• If pregnancy is suspected but a urine test is negative, waiting 2-3 days before retesting or seeking a blood test can provide more definitive results.
• False negatives are more common than false positives, particularly when testing is done too early.
• The risks of not starting to use contraception should be weighed against the risks of initiating contraception use in a woman who might be already pregnant, and most studies have shown no increased risk for adverse outcomes, including congenital anomalies or neonatal or infant death, among infants exposed in utero to certain contraceptive methods 1.

From the Research

Urine vs Serum BHCG Testing

• The studies provided do not directly compare urine and serum BHCG testing in terms of how early they can detect pregnancy 2, 3, 4, 5, 6.
• However, study 6 mentions that a commercially available quantitative beta-human chorionic gonadotrophin (BHCG) point of care testing (POCT) device, which uses whole-blood samples, performs comparably to gold standard laboratory methods that use serum samples.
• The POCT device had a correlation coefficient of 0.96 with laboratory BHCG values, indicating a strong linear relationship between the two methods 6.
• Study 3 found that the lowest beta-hCG concentration at which an intrauterine pregnancy was correctly diagnosed using vaginosonography was 141 mIU/ml, but the study does not mention urine BHCG testing.
• Study 5 found that serum beta-HCG levels were significantly lower in abnormal pregnancies, but the study does not compare urine and serum BHCG testing.

Detection Limits

• Study 3 found that all pregnancies associated with beta-hCG concentrations greater than 300 mIU/ml were correctly identified using vaginosonography 3.
• Study 4 found that transvaginal ultrasound was nondiagnostic in 67.2% of subjects with beta-hCG levels below 1500 mIU/mL 4.
• Study 5 found that serum progesterone levels were helpful in differentiating between normal intra-uterine pregnancy and abnormal intra-uterine or ectopic pregnancy, but the study does not mention urine BHCG testing 5.
• Study 6 found that the M6 model, which uses BHCG values to stratify pregnancy of unknown location as high risk or low risk, retained its performance when using POCT BHCG values 6.