Why does it take more than one pregnancy test to get 100% certainty? Why is one usually not trusted?
In: Biology
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.
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