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What Is An Abnormal Test Result?

Most teachers of evidence-based-medicine talk about tests as “positive, or negative”. A positive test is one in which the result of the test is abnormal; a negative test is one in which the test’s result is normal. A problem with this way of teaching about the value of test results is that often physicians and patients think there are only two possible test results, normal or not. However, test results are never just, “normal or abnormal”; test results may take on many values, not just two. ,

Researchers distinguish normal test results by performing the test in people who are well. For example, 100s of normal people will have blood tests done and the test results will vary over a narrow range. A serum potassium test result may be as low as 3.0 and as high as 4.0 in normal people, for example. An abnormal test result for potassium, then, is one whose value is greater than the highest in the range of values in normal people. But, the greater the potassium level, the more the diagnostic and treatment decisions may vary. In tesing, the magnitude of the result matters.

A key concept in testing is that the value of any test result may vary. The more abnormal it is, the more information it “contains” in terms of making a diagnosis. This may seem self evident, but failing to consider the absolute value of a test result is a common cause of missing the correct diagnosis in my experience.

The best way to understand this is to see an example. In the table below, I present a single test’s possible results. The test is PSA, or prostate specific antigen. It is a test used to find prostate cancer, but it is imperfect as the PSA test can be abnormal in diseases other than cancer.

If a physician, or you, just considers the test result as normal or abnormal, you will lose information about the value of the test. In the table, a high value (30, for example, in the first column of the table) means something different to you than a value of 20, or 10, or 5, even though all of those values are abnormal (any value greater than 2 in this example would be abnormal).

In the table, also, note that a test result value as high as 30 only occurs in people with cancer 1% of the time, which is a small percent chance. However, that level of the test never occurs in other diagnoses (in this example). Hence, a value of 30 means you have cancer. It is, in fact, a gold-standard test result at that level.

Any other abnormal test result value less than 30 in this example may increase the likelihood of having cancer, but those values do not mean for certain that you have cancer.

You can see from this table, then, that the actual test result value will have different meanings in terms of making a diagnosis. When we get to actual case examples in future blogs, you will see situations where a diagnosis was uncovered just by considering the information contained in the actual test result value.

My main point to you as a diagnostic decision maker is that you must know everything about your test results, including the exact value of every test’s results. Do not think of tests as just abnormal or not, know your test result values backward and foreword.

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rmcnuttBarry CarolHootsbudy Recent comment authors
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rmcnutt
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rmcnutt

Thank you for your comments. I am writing to patients with my blogs. I am an informed decision consultant and, hence, patients come to me with test results more often than with questions about if they should have a test. I am also a decision anaylst and understand the desire to “pre-think” testing. However, don’t see much evidence of thinking about whether to test or not. I did a study showing that the types and distributions of tests vary little by diagnosis in a hospital, for example. I see errors, however, when test results are considered “on average” or cut-points… Read more »

Barry Carol
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Barry Carol

Aside from the issue of who pays for the test, patients are much more willing to undergo tests that are not painful or invasive “just to be sure”. Imaging and simple blood draws fall into that category. They are probably much more hesitant to get tests that are painful or uncomfortable or involve unpleasant preparation. Moreover, patients may also think about the consequences of being wrong if they should decline tests like a PSA or mammogram and then find out too late that they have late stage cancer. There would undoubtedly be less testing if patients were paying more of… Read more »

Hootsbudy
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One of my favorite hobby horses. It has been clear for years that much of “testing” has more to do with trolling for business than protecting good health. Better health is, of course, a positive result, but too often the goal is hunting Easter eggs for the practice, courtesy of the insurance company. And besides, what patient will complain about the happy news that “we got (insert favorite target here) just in time”? How many false positives does it take to yield another case of prostate cancer? And of that number, how many are growing so slowly that the patient… Read more »