Medical Tests - What You Should Know

How valid are medical tests? - Even when a test is preformed correctly, the results may not be valid. Every test can be evaluated based on its sensitivity (the ability to detect a disease). No tests are 100% sensitive. But even tests with 80% sensitivity mean 20% of people having the tests will get the wrong results. For example, the stress electrocardiogram fails to show potential abnormalities or identifying existing heart disease in more than half of all tests! This is worse than a guess, which is theoretically correct 50% of the time.

The second criterion for evaluating lab tests is specificity. This percentage tells the test’s ability to show a negative or normal result when the condition does not exist in the person being tested. A test with an 80% specificity rating will give a false positive negative result 20% of the time; this means 20 out of every 100 people having the test will get a false positive results.

There are other statistical ways to look at tests. It’s most important to remember that no test is 100% accurate, so don’t have surgery or take an experimental drug or one with many nasty side effects based on one test.

How reliable are blood pressure, electrocardiograph and X-ray machines? - One study found that 1 out of every 2 blood pressure apparatuses gives the wrong reading, usually on the high side. Similar studies found that thousands of electrocardiograph machines are improperly calibrated. The other problem with electrocardiograms is that they and fail to detect heart problems in a great many patients (poor sensitivity) and falsely indicates heart problems in many other people (poor specificity), so it has a very low predictive ability and low accuracy. X-ray machines are often operated by improperly trained technicians that produce faulty or ineffective pictures.

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