special subjects
Medical Testing Decisions
Types of Medical Tests
Medical tests are done for different reasons, including Diagnosis Screening Disease classification and monitoring Some people may not benefit from tests (see Does Everyone Need a Test? ). Diagnostic tests are done to find the cause of people’s symptoms. When choosing tests, doctors and other primary care practitioners take the following into consideration: Which diseases are most likely Risks of the tests Costs of the tests Ability of the person to access and undergo the test Accuracy of the tests Seriousness of the person’s symptoms and general condition The person's preferences The most likely diseases are identified based on the medical history and physical examination, taking into account each person's specific findings and medical background. In general, doctors prefer tests that are Less invasive and have lower risks (for example, a blood test rather than a biopsy) Widely available Relatively inexpensive Highly accurate Highly reliable However, not all tests have all of these favorable features. Doctors must use their experience and education to select the best tests to recommend for each patient. There are many reasons why a person may not be able to have the doctor's first choice of test for a certain disease. The following are just a few examples: People with claustrophobia might be unwilling to undergo Magnetic Resonance Imaging (MRI) (MRI) testing People with certain types of Artificial Pacemakers may not be able to undergo MRI People with kidney disease or allergies may be unable to receive injections of the contrast agents used for certain computed tomography (CT) or MRI tests People who live in remote areas might be unable to travel to centers where certain tests are available People may be unable to afford the cost of the tests In these cases, the doctor would recommend thenext besttest. The seriousness of the symptoms and a person’s general condition can affect the choice of diagnostic tests: For symptoms that are mild and not likely to be caused by a serious disease,a doctor might order fewer tests. If necessary, additional testing can be done later. For serious illness and symptoms that point to a disease that might result in immediate harm,a doctor might order many tests at once to determine the diagnosis sooner. If time is of the essence, a doctor might even recommend hospitalization. When less invasive tests (for example, a blood test or a CT scan) fail to reveal the cause of the disease, a doctor might recommend a more invasive test (usually a biopsy or surgical procedure) as the next step. In such cases, people should understand the risks and benefits of the proposed procedure before agreeing to it, that is, they must give Informed Consent . In some special cases, a doctor also may ask people whether it is important to them to know if they have a certain disease or to get treatment for that disease if they do have it (see Defining goals ). In short, no. Although many people find medical tests reassuring, test results are not always right: Results are sometimes normal in people whohavea disease (false negative). Results are sometimes abnormal in people whodo nothave a disease (false positive). To test or not to test?The possibility of getting a false-positive result can make testing a poor idea. When the probability that someone has a disease is lower than the probability that the test for that disease will be falsely positive, the test is likely to be misleading. An example:Suppose parents are concerned that their 4-year-old daughter might have a urinary tract infection (UTI) because she is walking holding her thighs together. However, in the office, the doctor discovers that the girl has no other signs that suggest she has a UTI. That is, the girl is not urinating more frequently, she does not have pain or burning with urination, and her bladder and kidneys are not tender. Based on these findings, the doctor concludes that the likelihood of a UTI is very low (no more than 5%) and reassures the parents that nothing needs to be done unless other symptoms develop. The parents say they would feel better if the doctor did a urine test to prove their daughter did not have a UTI.Would a test help or hurt? Evaluating the potential usefulness of test results:Suppose the doctor did a test for UTI that was known to give false-positive results 10% of the time (10% false positive is typical for many medical tests). Even assuming that the test was always positive when peopledidhave a UTI, that means that in 100 little girls like this one The 5 who actually had a UTI would have atrulypositive test. But 10 would have afalselypositive test. In other words, inthis particular little girl, a positive test result is twice as likely to be wrong as it is to be right. Impact of test results on decision making:Thus, in this case, even a positive test result should not change the doctor’s decision not to treat, because that positive test result is likely to be wrong.Because the doctor would not do anything different, it makes no sense to do the test in the first place. It would be a different story if the doctor thought the likelihood of a UTI was higher. If the likelihood were 50-50, most of the people with a positive test result would actually have a UTI, and testing would be helpful. This math helps explain why doctors try to do tests only when there is a reasonable probability that people have the disease for which they are being tested. Screening tests are used to try to detect a disease in people who do not have symptoms. For example, most primary care practitioners recommend that all people over 45 have a colonoscopy to look for colon cancer even if they have no symptoms and are in good health. Screening is based on the natural idea that outcomes will be better if a disease is recognized and treated in its early stages. Although logical enough, this idea is not always correct. For some diseases, such as testicular cancer and ovarian cancer, there does not seem to be any difference in outcome between people whose disease is detected by screening and those whose disease is diagnosed after the first symptoms appear. A further potential problem with screening tests is that the results usually require confirmation by a more definitive test. For example, women who have a mammogram with abnormal results may need to have a breast biopsy. Such definitive tests are often invasive, uncomfortable, and sometimes a bit dangerous. For example, a lung biopsy can cause a collapsed lung. Because results of screening tests are sometimes abnormal in people without disease (which is common because no test is 100% accurate), some people undergo an unnecessary test that might harm them. Therefore, doctors recommend screening tests only for those diseases for which doing screening tests has been proven to improve outcomes. Clinical trials are necessary to tell which screening tests are effective and which people should undergo them. Despite these concerns, it is clear that for some diseases, such as high blood pressure and cervical cancer, screening saves lives. To be useful, tests used for screening must Be accurate Be relatively inexpensive Pose little risk Cause little or no discomfort Improve outcomes A screening test may not be appropriate if early treatment does not make a difference in the outcome of the disease being screened for or if the disease is very rare. Some 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 done to determine the type of breast cancer and 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 done periodically in people who take thyroid hormone to treat hypothyroidism to determine whether the hormone dose best meets their needs. A decision about how often such testing is needed is made based on the person’s situation. 