A 31-year-old woman uses an over-the-counter (OTC) pregnancy test that is positive one week after a missed period. She is taking phenytoin for epilepsy. How should the nurse explain the most likely cause for obtaining false-positive results?

• A. Having an irregular menstrual cycle
• B. Using an anticonvulsant for epilepsy
• C. Taking the pregnancy test too early
• D. Being under too much stress at work

Answer: (B)

The main idea here is that home pregnancy tests work by detecting human chorionic gonadotropin (hCG) in urine using antibodies. Sometimes medications can interfere with these antibody-based tests and produce a positive result even if pregnancy isn’t present. Phenytoin, an anticonvulsant, can alter how the test’s antibodies react or affect urine chemistry in a way that leads to a false-positive result. So, when a patient on phenytoin has a positive test one week after a missed period, the most plausible explanation is interference from the anticonvulsant rather than actual pregnancy alone. To clarify, a false-positive is not commonly caused by irregular menstrual cycles, taking the test too early (which more often yields false negatives), or stress; those factors don’t reliably produce a positive result. Advise confirming with a serum hCG test or trying a different brand of test, and review the patient’s medications with a clinician to determine the best next steps.