Medical Test Reliability


When you get a medical test, you may be anxious about the results. For the most part, medical tests are helpful. But most tests are not 100 percent reliable, and the result of any single diagnostic test is not usually enough to make a diagnosis without looking at the big picture.

Diagnostic tests include blood tests, urine tests, imaging tests, and special tests such as electrocardiogram (EKG) and electroencephalogram (EEG). Each test you have has a different rate of accuracy, and your healthcare provider can generally give you an idea of the reliability of the test. Often, the report itself includes a statement about the accuracy of the test.


Types of Inaccurate Medical Test Results

There are several causes and types of inaccurate medical test results. The accuracy of a test depends on a number of factors, including the range of normal values, the rate of false results, and whether you have to follow strict protocols, such as fasting from food before taking the test.

When tests have been available for many years, their accuracy rates are well known. For newer tests, the accuracy may not be well established.

Logistical Errors

Logistical errors such as paperwork mix-ups or having the wrong test may occur. If you were sent for a chest X-ray but had an abdominal X-ray, then your healthcare provider simply won’t have the right information.

Procedural Mistakes

Procedural mistakes such as incorrect handling of samples or inaccurate equipment calibration can occur, altering your results and making the report unreliable.


You may have circumstances that alter your test results—for example, if you are temporarily taking systemic steroid medications, your blood pressure and blood glucose are both likely to be elevated. While these results may be accurate at the moment of your test, they would not be reflective of your blood pressure and blood sugar, and the results should not be the basis of long-term treatment decisions.

Limited Information

The tests themselves may provide limited information. For example, an X-ray may show a mass in the lung, but further testing (such as a biopsy) may be needed to determine whether it is cancer, a benign tumor, or due to infection. As well, the chest X-ray can miss instances of lung cancer in 20% of cases where the person is showing symptoms.

False Negative

False negative results occur when a test result indicates that you do not have a medical problem when you actually have it. For example, high levels of thyroid antibodies can indicate thyroid cancer, but it is very common to have thyroid cancer without having abnormal thyroid antibody levels.

False Positive

False positive results occur when a test indicates that you have a medical condition even when you don’t really have it. For example, your blood pressure may be elevated if you are nervous about your healthcare provider’s visit, even if you don’t have high blood pressure. This condition, called white coat hypertension, is a type of false positive result.

Questions to Ask About Your Medical Test Results

Sometimes, you need a combination of several different tests, or you may need to have the same test run again to verify the findings. Your healthcare providers and nurses are familiar with the reliability and accuracy of most diagnostic tests. However, if you are having a rare test, such as a genetic test for a rare disease, this may not be the case.

When you are being evaluated due to symptoms such as urinary frequency, which is a symptom of diabetes, other factors besides a single blood sugar measurement are used to determine whether you have the condition or not.

When you are having a screening test, such as a prostate-specific antigen (PSA) for prostate cancer, false results can be more difficult to interpret because symptoms are not usually present in the early stages of the illness. Mammograms are another common screening test known for having high rates of false negatives and false positives.

Some questions you can ask your healthcare provider about your test results include:

  • Is this result consistent with your symptoms and your other test results?
  • How reliable is this test?
  • Do you need to have this test repeated and, if so, how often?
  • Are there other tests that can verify whether you have (or don’t have) the medical condition?

Keep in mind that when you have an at-home test without your healthcare provider’s order or involvement, you should be sure to research the accuracy and reliability of the test on your own.

A Word From Get Meds Info

Most diagnostic tests are not intended to be viewed in isolation—each test is just a part of your medical evaluation and should be considered in light of your overall health. Often, it is a change in your test results over time that provides the most information, rather than a single value or reading.

One of the most important things you can do as an empowered patient is to get copies of your test results so that you can have results to compare to at a future time, even if you switch your insurance or primary care healthcare provider.

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