Interpreting The Results Of Laboratory Studies: Be Careful
A few weeks ago, I read an interesting piece in the New York Times about errors that a national reference laboratory had made in measuring Vitamin D levels. Apparently the laboratory had recently discovered (or had been found out?) that for some time it had been reporting erroneous results for serum vitamin D levels, resulting in either falsely high or falsely low values. The problem was traced to inaccurate standards used in the assay. Anyway, the report got me to thinking about just how difficult it can be for a patient to have any confidence that a test his doctor orders will provide an accurate result, let alone be interpreted correctly by the doctor. I do know a little bit about such things given my background as both a clinician who has ordered lots of laboratory tests and as a former director of a large reference laboratory.
Obstacles to getting an accurate test result
Many patients and health care professionals assume that the mere ordering of a laboratory test is just about all the assurance they need that the test will be performed properly and that the result will provide useful clinical information. Nothing could be further from the truth. At every step of the way, from procurement of the specimen, be it blood or urine or whatever, to the actual test report, things can and do go wrong. For example, if the specimen was supposed to be collected with the patient in the fasting state but the patient had recently eaten, right away the meaning of the test results should be questioned. If the specimen is not collected, transported, and stored properly before analysis, serious errors can be introduced. If the assay is not performed properly, the results may be erroneous. If the standards used to normalize the test results are wrong, the results will be erroneous. If the normal range used in the test report has been improperly constructed, the test results might be interpreted incorrectly. If the test results reported for one patient are really those of a different patient, the results will be erroneous. Finally, if the health professional does get an accurate test result but interprets the result incorrectly, as far as the patient is concerned, it’s almost as bad as an erroneous test result. In summary, there are many places between the ordering of a test and the reporting of the test result that things might go wrong.
Don’t get me wrong, I believe that most laboratory tests are carried out with great skill and that the results are accurate. But one should never assume so and wise clinicians know that test results must be interpreted in context; if a test result does not fit with the clinical history, the result should be viewed with suspicion or at least skepticism as to its clinical significance.
Improving the odds that test results will be accurate
There are standard procedures called quality control/quality assurance that all laboratories are supposed to carry out to optimize test results. These might include running certified standards according to a specified protocol and participating in an external quality assurance program such as those carried out by the College of American of Pathologists or the NGSP (the organization that runs the standardization program for hemoglobin A1c, the test that is widely used in diabetes). Still, in the end it comes down to the health professional who ordered the test, using clinical judgment to interpret the test result, always reserving the right to question whether the number on the test report fits with the clinical impression.
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