Decisions About Medical Tests
Tests are performed to screen for disease, diagnose disease, classify and measure the severity of disease, and monitor the course of a disease, especially its response to treatment.
Screening uses tests to try to detect a disease when there is no evidence that a person has the disease. For example, measuring cholesterol levels helps determine the risk of cardiovascular disease in people who have no symptoms of cardiovascular disease. To be useful, tests used for screening must be accurate, be relatively inexpensive, pose little risk, and cause little or no discomfort.
Diagnosis, on the other hand, uses tests to confirm or rule out a disease when there is suspicion that a person has the disease. For example, a doctor who suspects serious heart disease might recommend cardiac catheterization. This test would not be a good screening test because it is expensive, can produce side effects, and is uncomfortable. However, these drawbacks are outweighed by the need for this test when the presence or absence of disease must be confirmed.
Tests are used to classify and measure the severity of a disease that has already been diagnosed. Results may lead to more specific and effective choices for treatment. For example, after a diagnosis of breast cancer is confirmed, additional tests are performed to determine if and where the cancer has spread.
Tests are also used to monitor the course of a disease over time, often to determine the response to treatment. For example, blood tests are performed periodically in people with insufficient production of thyroid hormone (hypothyroidism) to determine whether they are receiving a dose of replacement thyroid hormone that best meets their needs. A decision is made about how often such testing is needed for a person.
When deciding whether to recommend testing for a disease, especially for diagnosis, the doctor estimates how likely it is that a person has the disease. In coming up with an estimate for a specific person (the pre-test probability of disease), the doctor may consider information about the disease in the area where he practices, including how common the disease is (prevalence) and how many new cases of the disease occur during a specific period of time (incidence). The doctor also considers whether the person has particular characteristics (risk factors) that make him more or less likely to be affected. With this information, the doctor can then select the best test to confirm the presence of the disease.
When deciding whether to perform a test, a doctor must determine what its results may mean. Unfortunately, tests are not perfect. They may sometimes produce normal results for people who have the disease being tested for; that is, they produce false-negative results. They may sometimes produce abnormal results for people who do not have the disease being tested for; that is, they produce false-positive results. Therefore, important characteristics of a test are its sensitivity (the likelihood that it will produce abnormal results for people with the disease being tested for) and its specificity (the likelihood that it will produce normal results for people without that disease). A doctor can mathematically combine the pre-test probability of disease with the results of the test and with information about the test's sensitivity and specificity to come up with a more accurate estimate of the likelihood that the person has the disease (post-test probability).
Another characteristic of a test is its reliability. A highly reliable test gives the same result when a person undergoes the test more than once, unless the disease being tested for has actually improved or worsened. Results from a less reliable test may change randomly.
Before performing a test, a doctor weighs the potential harm of the test against the potential benefit of the information the test may provide. He must also consider how the results will be used. It may not be useful to perform a test if the results will not change the treatment that is recommended. For example, if a test is being considered to determine if a particular treatment would be an option for a person, but the person has already decided he does not want that treatment, then the test need not be performed.
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