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A a false-positive pregnancy test likely after taking Plan B? #111/9
I've just finish taking Provera for ten days about a week ago and during that time I had sex with my husband twice at two different times and he came inside me each time by accident.  Each time I was told by my doctor to take Plan B because I have to get a D&C done to remove some fibrods in my uterus. So until I had the all-clear from my doctor to start trying again, I didnt want to get pregnant again and risk another miscarriage. I took a pregnancy test today and it came back positive! Is there a possibility that it my be a false positve?

What contraceptive have you been using? 


False positive tests are less likely than false negative tests.  Your history demonstrates the problem with withdrawal and explains why the perfect user failure rate for withdrawal, 4% over the course of an entire year is so much lower than the typical use failure rate for a year, 27%.  In actual practice, the man wants to continue after his orgasm, the woman wants to continue after his orgasm, or both.  This raises the typical user failure rate compared to the perfect user rate, which is really quite respectable.


Here is extensive information from a chapter on pregnancy testing in Contraceptive Technology:


Accuracy. Test accuracy can be affected by the techniques and experience of the user, and by the user’s ability to follow the test instructions precisely. The most common error with home pregnancy tests is a negative result that occurs because the test was performed too early in pregnancy. An incorrect result could mislead a woman, causing her to delay in getting a clinical evaluation.


Uses. If a home test result is positive, positive results may prompt a woman to seek pregnancy confirmation earlier than she otherwise might and to change her lifestyle earlier than were she to wait for a clinical evaluation. Clinical evaluation is needed to confirm the pregnancy, determine the length of gestation, and identify any possible risk for ectopic or abnormal pregnancy. If the home test result is negative, clinical evaluation also may be needed to determine the cause of menstrual delay or of the other symptoms that have prompted the test, especially if the woman does not resume normal menses soon.  Avoiding Pregnancy Test Interpretation Errors

Correctly interpreting pregnancy test results is not entirely straightforward because:

1.   hCG levels change drastically over the course of pregnancy, especially during the first 8 to 10 weeks (Fig­ure 23-2).


2.   Both positive and negative test results must be interpreted in relation to the sensitivity, specificity, and characteristics of the particular test being used and the clinical evaluation findings, including ultrasound examination when appropriate.


If pregnancy test results do not agree with other clinical signs, consider the possible reasons for the discrepancy. Plan appropriate follow-up or further evaluation to protect the patient against possible consequences of an incorrect test result:

1.         Any test result can be wrong. Laboratory errors do occur, includ­ing specimen mix-up and incorrectly performed test procedures. For accurate results, instructions for the kit must be followed meticulously and timed with a stopwatch. Use control solutions to verify accuracy. Observe test-kit and reagent expiration dates.


2.         Know exactly what kind of pregnancy test was performed and what sensitivity the test has. Without this information, it is not possible to assess the clinical significance of a negative result or to evaluate the possibility of a false positive result.


3.         Do not base clinical management on the results of a home pregnancy test. Although home kits have excellent theoretical accuracy, their use even by trained personnel may not reliably provide the sensitivity or specificity needed for optimal clinical management.  Be careful about accepting the results of a pregnancy test performed in another facility, especially in critical clinical situations such as ectopic pregnancy.


4.         False negative results are common with less sensitive urine pregnancy tests. False negative results frequently occur because the test is performed too early or too late in pregnancy. Abnormal pregnancy, urine that is too dilute, and medication that interferes with the test result may all be responsible. Use a highly sensitive urine pregnancy test to “rule out” pregnancy.


5.         False negative results are rare with sensitive urine tests but can occur if test procedures are performed incorrectly or if the test reader has red-green color blindness. A highly sensitive urine pregnancy test also may be negative if the test is per­formed too early in the cycle, before implantation occurs. In this situation, the result is a “true” negative, which could be mislead­ing if not repeated a few days later. Elevated lipids, high immuno­globulin levels, and elevated urine proteins associated with severe kidney disease also can interfere with a test result. If a false negative result is suspected, order a serum quantitative beta hCG.


6.         It is rare but known in the literature that urine and serum pregnancy tests may yield false negative results when hCG is present at extreme concentrations. This is known as the “high-dose hook effect.” The secondary antibody binding sites become saturated with free hCG so that the primary antibody complex is unable to bind, preventing detection of the antigen. In one case of molar pregnancy, only after multiple dilutions of the serum was a beta hCG of 1.5 million mIU/ml detected.  The important thing to remember is that laboratory test results should be questioned if they are discordant with clinical findings.


7.         False positive pregnancy test results are not common, but they can cause perplexing dilemmas: 


           False positive results with highly sensitive urine pregnancy tests are very rare, but laboratory error is always possible. An “accurate” false posi­tive result could occur if the woman has had treatment involv­ing hCG injection within the preceding 14 days, and faint false positive results have been reported with urine samples contaminated by blood or recent use of Chinese herbal medication. If a false positive result is suspected, obtain a serum quan­titative beta hCG.

           If a less sensitive urine pregnancy test is positive, but the uterus is not enlarged, perform a confirmatory highly sensitive urine pregnancy test. The positive result could be caused by LH cross-reaction, in which case the highly sensitive urine pregnancy test will be negative be­cause it is specific for beta hCG.

           Less sensitive urine pregnancy tests (agglutination slide tests) also can yield false positive results because of protein or blood in the urine specimen. Consider obtaining a confirmatory highly sensitive urine pregnancy test if the urine specimen shows 1+ proteinuria or more. A highly sensitive urine pregnancy test is likely to give an accurate (negative) result. When a positive pregnancy test is not confirmed by the presence of a pregnancy in the uterus, do not assume the test result is false. Seriously consider the possibility of an ectopic pregnancy. 


8.         In very rare cases, pregnancy test results are positive even though the patient is not pregnant, because hCG actually is present and originating from a source other than pregnancy. hCG levels persist after a recent pregnancy or after hCG treatment. Low levels of hCG (5 to 30 mIU) may be associated with tumors of the pancreas, ovaries, breast, and many other sites. Some normal postmenopausal women also have low levels of circulating hCG-like substance of pituitary origin.

Has it turned out that you are pregnant?  Please let us know.


Key Words: Provera, sex, ejaculated, accident, Plan B, D&C, fibroids, uterus, pregnant, miscarriage, positive pregnancy test, false positive, negative, Contraceptive Technology


Fjerstad M, Stewart F. Pregnancy testing and management of early pregnancy IN Hatcher RA, Trussell J, Nelson AL. et at Contraceptive Technology 19th edition, page 606: Ardent Media Inc. 2008

Posted 1-19-2009, Updated 1-27-2009, Updated 1-30-2009, Updated 2-6-2009 

Robert A. Hatcher MD, MPH
Emeritus Professor of Gynecology and Obstetrics
Emory University School of Medicine
Atlanta, GA

Managing Contraception for Your Pocket 2013-2014

Managing Contraception for Your Pocket 2013-2014

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