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Dr. Andrew Rynne
MD
Dr. Andrew Rynne

Family Physician

Exp 50 years

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Why Do Pregnancy Test Results Vary When Checked By Different Test Strips?

Hi! I had my copper IUD taken out on 1-4-18. The first day of my last period was 12-28-17. Husband and I had sex on the 5th and the 10th. I’ve been having pregnancy symptoms. On the 25th I took a preg test which came back positive. On the 26th I took 3 more which also came back positive. This morning I took another test and it came back negative. I ran out and bought the first response test and confirmation test kit. I just used the regular stick and that also came back negative? Why would this happen?
Fri, 23 Feb 2018
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General & Family Physician,  's  Response
Hello and thank you for using HCM for your questions!

Short answer: You will always get different results for the same parameter when using different tests!

Detailed answer: Any test result is based on the ability of it to distinguish correctly the sicked (or healthy) individuals. The test result itself will depend on the threshold used. For example, if normal blood pressure is 120/80 then I can choose that my test will consider hypertensive those persons with BP >=140/90. Another test might consider hypertensive those persons with BP >=130/85, etc. Therefore, each test will give different results based on the cut-off values it uses. There are different cut-offs for everything because health authorities do not agree on the level beyond it a person is considered as being affected by the health event. This is also true for pregnancy tests. Therefore, it is possible that different tests will yield different results.
In your case you might have gotten a true result (really pregnant or really not-pregnant), a false-positive result (you are not pregnant but the test tells you are) or a false-negative result (you are but test tells you are not). Quick pregnancy test are not very accurate, because in some cases they miss-classify individuals (as in your case). It is not possible, in your case, to determine which test result is the correct one.
Therefore, you need to undergo some other examinations (echo, for example) or other more sensitive tests to determine your pregnancy status.
I hope I have answered your questions and wish you good luck!
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Why Do Pregnancy Test Results Vary When Checked By Different Test Strips?

Hello and thank you for using HCM for your questions! Short answer: You will always get different results for the same parameter when using different tests! Detailed answer: Any test result is based on the ability of it to distinguish correctly the sicked (or healthy) individuals. The test result itself will depend on the threshold used. For example, if normal blood pressure is 120/80 then I can choose that my test will consider hypertensive those persons with BP =140/90. Another test might consider hypertensive those persons with BP =130/85, etc. Therefore, each test will give different results based on the cut-off values it uses. There are different cut-offs for everything because health authorities do not agree on the level beyond it a person is considered as being affected by the health event. This is also true for pregnancy tests. Therefore, it is possible that different tests will yield different results. In your case you might have gotten a true result (really pregnant or really not-pregnant), a false-positive result (you are not pregnant but the test tells you are) or a false-negative result (you are but test tells you are not). Quick pregnancy test are not very accurate, because in some cases they miss-classify individuals (as in your case). It is not possible, in your case, to determine which test result is the correct one. Therefore, you need to undergo some other examinations (echo, for example) or other more sensitive tests to determine your pregnancy status. I hope I have answered your questions and wish you good luck!