Why does it take more than one pregnancy test to get 100% certainty? Why is one usually not trusted?
In: Biology
A huge part of the problem is that you unpack an indicator intended for one time use, that straight out of the package indicates NO. And after you’ve taken the test, and it still says NO, how can you be sure that it’s the correct answer?
Did you do the test correctly? Is it working correctly? (was it manufactured correctly?)
And at this point, we are only talking about if you get a false negative. If it incorrectly tells you that you are not pregnant.
In the next step, you have to address all the things that could give a false positive; the things that could suggest that you are pregnant even though you are not. This includes rare medical conditions that could fool the test. And again, manufacturing issues. And use that goes against the instructions.
With every test, you have some probability of false positives occurring (the test shows positive, but the condition isn’t actually present) and some probability of false negatives occurring (the test shows negative, but the condition actually is present). In a perfect test, both of these probabilities would be 0, but perfect tests are generally impossible, expensive, and/or intrusive.
Now, we can usually design the tests to try to minimize one type of false result, but doing so usually increases the other type. That is, if you change the test so that fewer false negatives occur, those changes will typically cause more false positives to occur. You can’t have your cake and eat it too.
In medicine, it’s generally better for a test to minimize the false negatives (telling the patient that they’re fine when they actually have a condition) than to minimize the false positives (telling the patient that they have the condition when they’re actually fine). This is because medical conditions tend to get worse over time and people generally don’t seek more tests if the first one says they’re fine, but more tests is the only way to detect whether the first result was erroneous or not.
Because they’re not 100% accurate. If they’re 90% accurate (for example) and you just take one, you have a 90% chance of being right and 10% chance of being wrong.
If you take a second test and it says the same as the first, you now have a 99% chance of being right and a 1% chance of being wrong
And so on
It’s all about statistics: the more tests you take that all say “positive,” the higher the chance that you’re actually pregnant.
It’s often a factor of time. Pregnancy tests are remarkable accurate in identifying the pregnancy hormone, HCG. However, if there is too little HCG in the bloodstream, the test won’t pick it up.
HCG levels rise very, very quickly in the first few weeks of pregnancy, so a test might be negative when the pregnancy is very new, but positive a few days later.
If your test is positive: no need for a retest
If negative, wait for 1-2 days and test a second time.
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