Pregnancy testing relies upon the detection of the beta subunit of human chorionic gonadotrophin in the mother's urine after overnight concentration. The test is usually positive from the first day of the last missed period.
With the advent of monoclonal antibodies, pregnancy tests are now more specific and sensitive. Sensitivity to a level of 25 IU hCG/litre is great enough to exclude ectopic pregnancy in combination with history, examination, ultrasound and / or laparotomy.
False positive results can occur if pituitary gonadotrophin levels are elevated in the urine, for example, in older women when ovulation is failing and the pituitary excretes more gonadotrophins to stimulate the failing ovary. Haematuria, alkaline urine and vaginal discharge can also affect the test, but these false positives are decreasing with improved specificity of assays.
False negatives can occur if the test is done too early in pregnancy since their is insufficient chorionic gonadotrophin.
Infrequently, ultrasound can be used to diagnose pregnancy in doubtful cases as the embryonic sac is visible at six to seven weeks.
Note that a pregnancy test may remain positive for about five days after fetal death or abortion.
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