Pregnancy test: False negatives on home pregnancy test

Why is it that some people test positive the day they miss their periods, while other women don't test positive until they are a few days to a week late?

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Peg Plumbo CNM

Peg Plumbo has been a certified nurse-midwife (CNM) since 1976. She has assisted at over 1,000 births and currently teaches in the... Read more

Women produce varying levels of hCG despite similar gestational ages. It is not always a given that the embryo implanted precisely at 14 days after the first day of the last menses. Fertilized ovums have been known to drift in the uterine environment for days before implanting. This would affect hCG levels and timing of positive tests.

There also are many sensitivities attributed to the various pregnancy test kits; some pick up as little as 25 mIU/ml with the average being a sensitivity of 300 mlU/ml, which is the standard recommended sensitivity in detecting the average pregnancy 35 days after the last menstrual period. According to manufacturers, all tests will detect pregnancy in 100 percent of pregnant women within 42 days after the last menstrual period. (Note: study results valid for women with a 28 day cycle.)

Test kits are not required to provide data showing the sensitivity and specificity of their product, so it can be difficult to make informed choices between products on the pharmacy shelf.

Sixteen kits were evaluated in one large analysis. This study found that when women tested pre-collected samples under study conditions in a lab, accuracy was 91 percent. When they collected their own specimens at home, accuracy fell to 75 percent.

About one-third of all women in the U.S. have used a kit before seeking professional advice, which has led to an earlier diagnosis. This has great advantages to women who subsequently avoid risky behaviors and improve nutritional intake very early in their pregnancies.

False positive tests are very uncommon, especially with the newer tests (using monoclonal antibodies to hCG). False negative results are more common and are especially undesirable because they delay onset of care for women who may then use potentially hazardous medications after being falsely reassured that they are not pregnant. The primary cause of false negative pregnancy tests is inaccuracies in timing of the test or in the procedure itself. Following kit instructions to the letter is very important but even then false negatives do occur. The test should be repeated weekly if the woman still does not have a period.

As home pregnancy tests are refined, accuracy (the rate of false positives and false negatives) continues to improve. Home testing for pregnancy, HIV and a variety of other sexually transmitted diseases is big business and millions of dollars are invested in product development, improvement, advertising and consumer support.

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