Urine Pregnancy Test

Introduction

Urine pregnancy testing is the most common method for diagnosing pregnancy.

  • A variety of affordable and reliable qualitative urine tests are available.
  • It takes only 1-5 minutes to perform.
Women with irregular cycles or an uncertain last menstrual period should generally wait at least 14 days from the most recent sexual act before obtaining a pregnancy test.



Urine Sample

A random urine sample can be used for testing because hCG production is not circadian.

  • The most common cause of a false negative result is performing the test too soon after conception. The test is negative because ovulation (and thus fertilization, implantation, and first day of the missed menstrual period) occurred later than expected.
  • Rarely, false negative results are due to a "hook effect". When a very high hCG concentration is present and the sample is tested without prior dilution, both the capture and tracer antibodies used in immunoradiometric assays become saturated, preventing the binding of the two to create a sandwich. Since the non-sandwiched tracer antibodies are washed away with the excess material, the test result will be negative. This is most commonly seen with the very high hCG levels associated with gestational trophoblastic disease; hCG levels are generally lower in normal pregnancies.
  • An assay's inability to recognize specific isoforms of hCG can also play a role in false negative results.

If pregnancy is suspected despite a negative test, the test should be repeated in one week. Waiting a week or two after a missed period before performing a urine pregnancy test not only minimizes false negatives, but also decreases the tendency to perform a serum hCG test to exclude or confirm very early pregnancy after a negative urine test.



False-Positive Test

False-positive pregnancy tests are rare and due to

  • Operator error in performing or interpreting the test.
  • Biochemical pregnancy (i.e. pregnancy loss very soon after implantation and before signs of pregnancy are apparent on ultrasound).
  • Exogenous hCG administered as part of infertility treatment or for athletic performance. Exogenous hCG should be cleared by 2 weeks post-injection.
  • hCG secretion from a tumour.
  • Pituitary hCG secretion, typically in perimenopausal women.
  • Interference with the assay by anti-animal antibodies, anti-hCG antibodies, or other substances (e.g., high doses of biotin serum test positive but urine test is usually negative).
  • Familial hCG syndrome.



Serum Pregnancy Test

In clinical practice, the most sensitive method for detecting hCG in early pregnancy is a serum pregnancy test.

  • Serum pregnancy tests typically detect hCG levels as low as 1 to 2 milli-international units/ml; a "negative" pregnancy test is reported as hCG <5 milli-international unit/ml. By contrast, urine pregnancy tests are less sensitive, detecting hCG beginning at a level of 20 to 50 milli-international units/ml.
  • In addition, the median hCG concentration is higher in serum than in urine.

Hence, very early in pregnancy, a serum pregnancy test may be positive while the urine pregnancy test is still negative.



Summary

Urine pregnancy tests are generally very accurate, but they are not 100% accurate.

  • It is important to remember that a negative test does not always mean that the woman is not pregnant.



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