False Positives and False Negatives – How Do I Know That My Pregnancy Test Result is Accurate?

When you’re wondering if you’re pregnant (whether it’s planned or not), the last thing you want is a inaccurate test result. This can lead to a lot of emotional upheaval, whether your pregnancy test displays a false positive or false negative. To make an informed decision on your next steps or to consider your options, it is important to know that your pregnancy test result is accurate. 

How does a pregnancy test work?

A pregnancy test measures the level (or absence) of human chorionic gonadotropin (HCG) in your urine, which your body begins to produce after the implantation of the pregnancy. Positive tests can be seen about 10 days after fertilization. 

How Accurate Are At-Home Pregnancy Tests?

Home pregnancy tests are generally considered to be accurate if taken correctly and at the right time. Around 10% of pregnancies don’t begin to produce a testable amount of HCG in the urine until a missed period, which means a false negative may result when testing too early. This is why Alpha Center does not schedule pregnancy tests before either a missed period or a positive pregnancy test. 

False Positive Pregnancy Tests

A false positive means that your pregnancy test gives a positive result (usually showing two lines in a traditional pregnancy test) when you aren’t actually pregnant. While pregnancy tests are usually considered to be reliable and accurate, there are a few rare circumstances that can lead to a false positive, including:

• You have recently experienced pregnancy loss (within the last 8 weeks)
• You have recently had an abortion (within the last 8 weeks)
• You have been taking fertility medicine that contains HCG
• You have ovarian problems
• Your test is faulty or you are taking a blue line test, which can cause evaporation lines

What is an evaporation line?

A pregnancy test that shows either a “+” or a double line to indicate pregnancy works by reacting with the HCG in your urine to change the color of the test line. HCG  occurs when a blastocyst (the stage of fetal development where implantation occurs) embeds into the uterine wall. The control line will appear regardless, indicating that the test works (If the control line does not appear, you should use a new test). However, if the urine test is left sitting for longer than the instructions recommend to wait before checking the result, an evaporation line may appear.

 An evaporation line is a colorless line that is caused by the liquid sitting in the test for too long.  Evaporation lines are frequently mistaken for a double line or positive test, but if it’s been longer than the test window recommends when this line appears, it is generally assumed to be a negative test. If this happens, you should take another test, following the test’s instructions. 

Why might I have a false positive?

• Human error – you may be mistaking an evaporation line for a positive result (as mentioned and pictured above), or you may have taken the test incorrectly.
• Chemical pregnancy – this happens when egg and sperm meet but the blastocyst fails to implant. 
• Certain medications – some medications like antipsychotics, anticonvulsants, antihistamines, or fertility drugs could cause a false positive.
• You’ve had a recent miscarriage or abortion –  Some HCG may remain in your system which can lead to receiving a positive pregnancy test up to 8 weeks following either a miscarriage or abortion
• Ectopic pregnancy – when the pregnancy implants inside the fallopian tubes rather than the uterus, a positive test can result. Because an ectopic pregnancy is outside of the uterus, it is not viable and needs immediate medical intervention. 
• Cancers – ovarian, bladder, kidney, liver, lung, colon, breast, or stomach cancer can all cause a false positive on a pregnancy test

Limitations of a Positive Test

A positive pregnancy test does not provide information on whether or not the pregnancy is located in the uterus or whether the pregnancy is viable. 

If you’re unsure what to do with your pregnancy or are considering abortion as an option, going to Alpha Center for a pre-abortion screening is a great start and will equip you with the information you need to make an informed decision about your pregnancy with your health in mind. 

False Negatives

What is a false negative?

Simply put, a false negative pregnancy test is a pregnancy test that reads negative when the user is in fact pregnant. There are a few common reasons for this. 

Why might I have a false negative?

• It’s too early to test: Every woman’s body is unique and, as a result, HCG levels can increase at different rates from person to person. This means that while two different women could be the same number of days pregnant, one could test positive and the other could test negative. The general rule is to wait to test until the day your period is due and doesn’t arrive. (However, this may still be too early- the author of this blog didn’t have a positive result until 5 days past her expected period!) If you have tested negative but are still experiencing symptoms, wait 48 hours and take another test.
• Dilute urine: As you hydrate throughout the day, your urine becomes more dilute and there may not be enough HCG to show up on a test, especially if it’s a digital test. For the most accurate results, take a test first thing in the morning when your HCG levels are most concentrated. 
• You checked the result too early: Follow the instructions that your test comes with – if you check at three minutes and the instructions say to wait 5 minutes, you’ve checked it too early! Especially early on in pregnancy, it may take longer for a positive result to show. 

If you’ve taken a pregnancy test with a negative result but your period still hasn’t arrived, test again in 48 hours to see if the result changes. Since HCG levels double every 48 hours in early pregnancy, it’s possible that your body hasn’t produced enough to react with the test strip. If you continue to get a negative test but haven’t had your period, make an appointment to see your doctor. 

Minimizing the Risk of False Results

To ensure that a pregnancy test gives accurate results, ensure that you are taking it at the right time in your cycle (after your period has been missed), avoiding diluted urine samples, and running the test according to the instructions. For further accuracy, we recommend using pink dye or digital tests. 

What Kind of Pregnancy Test Should I use?

To avoid an evaporation line and any confusion, consider either using a pink dye pregnancy test (the box should tell you what color of dye it uses).  A digital pregnancy test is easier to read but generally less sensitive to HCG, so if you are testing early it is more likely to have a false negative.

Following a positive pregnancy test:

A positive test can produce many emotions, including stress, joy, or both! 

If this pregnancy was unplanned, consider calling Alpha Center for a free pregnancy or pre-abortion screening appointment as you consider your pregnancy options. During this appointment, the important health information you need before having an abortion or moving forward with your pregnancy will be answered, including:

• Am I pregnant?
• Is the pregnancy viable?
• Is the pregnancy in the uterus or could it be ectopic?
• How far along am I?
• Do I have an STD?

The answers to these questions will provide information on the options you have and if you need to pursue further treatment or care. 

Further questions?

If you have more questions, call Alpha Center to schedule an appointment today at 970.239.2272, or if you suspect something more serious or complicated could be going on, call your primary care provider or go to the emergency room.

Note: This blog post is not meant to be a substitute for medical advice. Authored by Kristen Long and reviewed by Meghan Downing, RN & Nurse Manager for Alpha Center.

References

Early Pregnancy Diagnosis – StatPearls. (2023, January 2). NCBI. Retrieved October 10, 2024, from https://www.ncbi.nlm.nih.gov/books/NBK556135/

Home pregnancy tests: Can you trust the results? (2022, 12 23). Mayo Clinic. Retrieved 10 10, 2024, from www.mayoclinic.org

Home pregnancy tests: Can you trust the results? (2022, December 23). Mayo Clinic. Retrieved October 10, 2024, from https://www.mayoclinic.org/healthy-lifestyle/getting-pregnant/in-depth/home-pregnancy-tests/art-20047940

Kay, C. (n.d.). False-positive pregnancy test: 5 possible causes. MedicalNewsToday. Retrieved October 10, 2024, from https://www.medicalnewstoday.com/articles/319803#accuracy

Perry, C. (2024, January 24). Evaporation Line on Pregnancy Test: Causes, How to Avoid. The Bump. Retrieved October 10, 2024, from https://www.thebump.com/a/evaporation-line-on-pregnancy-test

Wilson, D. R., AHN, B., & Petre, A. (2020, June 2). Are Pink Dye Pregnancy Tests Better? Healthline. Retrieved October 10, 2024, from https://www.healthline.com/health/pregnancy/pink-dye-pregnancy-test#takeaway