Testing for any type of medical condition has a certain degree of inaccuracy. No matter the test, sometimes patients will have a false positive result, suggesting they have a condition when they don’t. Alternately, a test may result in a false negative, where the test fails to return the positive result it should. While false positive tests might result in treatment for a condition that isn’t present, the false negative test may be, in certain circumstances, more serious because people may not get needed medical care.
There are many potential reasons that a false negative test could occur, and these reasons depend on the type of test being performed. Sometimes the issue is sample collection size — there was simply not enough blood, urine, or tissue to accurately perform the test. At other times, a test is performed too soon, and whatever the condition, it hasn’t had enough time to build up the results that would suggest positive testing. Very early pregnancy tests and early HIV testing may yield false negatives. Results may be rechecked, especially with HIV, a few months later to make sure the result was not a false negative test.
Different forms of testing dictate what a predictable result is for the particular condition being tested for. A blood test might screen for a specific enzyme, protein, or hormone. If it is not found, the test might yield a false negative test result because sometimes patients manifesting a condition lack the symptoms the test is designed to catch. For example, some pregnant women have low levels of HCG well into their pregnancy, and this could yield many false results. When pregnancy is diagnosed through other means, like ultrasound, it also raises concern about the health of the pregnancy and fetus.
The possibility that an atypical response will yield a false negative test leads to different methods of backup testing or a range of diagnostic criteria. Instead of only relying on a single test, doctors have a variety of ways to look for and diagnose disease. Ultrasound, for instance, can confirm a pregnancy when blood test results don’t. This backup testing is useful, but isn’t available with every disease or condition, or diagnosis might not be available until a condition has advanced past a certain stage. There are plenty of false negative Pap smear results, but diagnosing cervical cancer early by other means can be challenging.
Most patients receiving a medical test will get accurate results. Those concerned about outcome can ask about the test’s accuracy. If they receive results they believe represent a false negative, they could continue to pursue this with physicians. Sometimes simply repeating a test is enough to determine whether it has been truly accurate.
