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What medication can cause false negative pregnancy test?


A pregnancy test detects the presence of the human chorionic gonadotropin (hCG) hormone in urine or blood. This hormone is produced by the placenta shortly after implantation occurs. A false negative pregnancy test occurs when the test fails to detect hCG, even though the person is pregnant. There are several reasons why a false negative can occur, including very early pregnancy, improper test usage, diluted urine sample, and certain medications.

Some medications can interfere with pregnancy test results and cause a false negative. The most common culprits are hormones that contain hCG-like substances. When present in the urine, these substances can cross-react with antibodies in the test and prevent the detection of true hCG. Other mechanisms by which medications cause false negatives include delaying implantation, decreasing hCG production, or increasing hCG clearance from the body.

Being aware of which medications can influence pregnancy test results is important for anyone trying to conceive or suspecting pregnancy. A false negative finding provides false assurance that pregnancy has not occurred. This article will review the medications known to cause false negative pregnancy tests and discuss the mechanisms by which they interfere with test accuracy.

Hormonal contraceptives

Hormonal forms of birth control are a major cause of false negative pregnancy tests. Birth control pills, injections, implants, patches and intrauterine devices (IUDs) contain estrogen and progestin hormones that inhibit ovulation and prevent pregnancy. Even though these contraceptives are highly effective when used properly, no method is 100% foolproof and unintended pregnancies can still happen.

The progestin compounds in hormonal contraceptives can have structures that mimic hCG. When present in urine, these similar molecules can occupy the test antibodies so that true hCG goes undetected. This leads to false negative results in women who conceive while on birth control.

The birth control methods most associated with false negative pregnancy tests are:

Birth control pills

Oral contraceptive pills contain estrogen and progestin. Normal-dose pills have enough hormones to reliably prevent ovulation. But even with perfect use, pills are about 91% effective because ovulation can occasionally occur. The progestin in birth control pills can cause false negative pregnancy tests by interfering with the assay.

Depot medroxyprogesterone acetate injections

DMPA, brand name Depo-Provera, is a contraceptive injection given every 3 months. It provides prolonged prevention of ovulation. However, its typical use effectiveness is only about 94%. False negative pregnancy tests may occur for several weeks after fertilization in women who ovulated in spite of Depo-Provera use.

Etonogestrel implants

Contraceptive implants, like Nexplanon, are matchstick-sized plastic rods placed under the skin that release progestin continuously for years. They are more than 99% effective, but pregnancy is still possible with irregular implantation. The etonogestrel hormone mimics hCG and causes false negatives.

Levonorgestrel IUDs

Intrauterine devices (IUDs) like Mirena contain progestin that is released locally in the uterus. Less than 1% of users get pregnant. But an IUD pregnancy may not be detected on urine tests due to interference from levenorgestrel.

Hormone therapies

Medications that contain reproductive hormones for purposes other than contraception can also lead to inaccurate pregnancy test results. The hormone compounds present can cross-react with test antibodies. Examples include:

Progesterone supplements

Progesterone is sometimes prescribed to support early pregnancy or treat infertility. Natural progesterone and synthetic progestins like hydroxyprogesterone caproate (Makena) have structures that resemble hCG. False negatives can occur during progesterone use in early pregnancy.

hCG injections

Human chorionic gonadotropin injections are used to trigger ovulation and stimulate follicle growth in fertility treatments. Directly taking hCG can lead to a false negative by overwhelming the antibodies in the pregnancy test. Very high hCG levels cause a “hook effect” that prevents detection.

Gonadotropin-releasing hormone (GnRH) agonists

GnRH agonists like leuprolide (Lupron) suppress estrogen and testosterone production. They may be used for endometriosis, uterine fibroids or prostate cancer. By inhibiting ovulation, GnRH agonists reduce the possibility of pregnancy. They can also delay implantation or decrease hCG levels, leading to false negatives.

Antibiotics

Certain antibiotics have been associated with false negative pregnancy tests by preventing implantation or interfering with hCG secretion:

Tetracyclines

Tetracycline antibiotics including doxycycline and minocycline have anti-inflammatory and anti-angiogenic effects. By inhibiting embryo implantation, they may cause false negative urine and blood tests in early pregnancy. However, tetracyclines are not considered highly teratogenic.

Quinolones

Ciprofloxacin and other quinolone antibiotics may prevent implantation or diminish hCG production by effects on embryonic development. False negatives are most likely within 1-2 weeks of conception. safer alternatives should be used when pregnancy is possible.

Other medications

Additional types of medication have also been implicated in false negative pregnancy tests:

Antipsychotics

Certain antipsychotic drugs like chlorpromazine and thioridazine have hCG-like structures. Use of these tranquilizers can cause false negative urine pregnancy results. However, these older antipsychotics have been replaced by more modern alternatives with less hormonal side effects.

Anti-epileptics

Some anti-seizure medications like phenobarbital, primidone and ethosuximide increase metabolism of hCG by the liver. This accelerated clearance from the body can lead to false negative blood and urine pregnancy tests.

Diuretics

Diuretics like furosemide (Lasix) cause increased urination and dilution of the urine. A diluted urine sample may fail to detect hCG, even when a woman is pregnant. False negatives are most likely with high diuretic doses.

Laxatives

Laxatives can also lead to liquid stools and concentrated urine. Diminished levels of hCG in the urine increases the chance of false negative results on over-the-counter pregnancy tests.

When to retest after a negative result

The possibility of a false negative pregnancy test result should be considered if conception occurred within the last 2-3 weeks. Another test should be performed with first morning urine a week after the initial negative test. If pregnancy is still suspected after a second negative result, a blood test is recommended for confirmation.

Quantitative blood tests detect hCG levels as low as 1-2 mIU/mL, while urine tests require at least 20 mIU/mL. Blood tests are thus more accurate and can identify very early pregnancies or rule out false negative urine tests.

Precautions when using medications

Certain precautions can help avoid false negative pregnancy tests due to medication use:

– Consider switching methods or go off birth control 1-2 months before trying to conceive. This allows time for medications to clear the body.

– Avoid medications that may cause false negatives when pregnancy is possible. Discuss options with your doctor.

– If you must take questionable medications, use condoms too until pregnancy is ruled out.

– Retest 1-2 weeks after an initial negative result if pregnancy is suspected. Get a blood test for confirmation.

– Inform your doctor about use of hormones, antibiotics or other medications when interpreting pregnancy test results.

Conclusion

Medications that contain hormones similar to hCG are a frequent cause of false negative urine and blood pregnancy tests. Hormonal contraceptives, hormone therapies, some antibiotics and other drugs can interfere with the accurate detection of hCG. Being aware of this possibility and retesting later can help avoid mistaking a false negative for a true negative result. When pregnancy is possible, caution is advised with any medication that could impair implantation or influence hormone levels. Both patients and doctors need to be informed about pharmacological causes of inaccurate pregnancy tests.