Because many different diseases can cause the same symptoms, it can be challenging for doctors to identify the cause. Doctors first gather basic information by talking to and examining the person. Often, this is all that is needed to make a diagnosis. At the very least, the basic information narrows the list of possibilities and limits the number of tests that need to be done. Ordering tests without first narrowing down the list of possibilities can be costly, increase the risk of an incorrect diagnosis, and expose people to undue risk (see Does Everyone Need a Test?).
(See also Overview of Medical Decision Making.)
Doctors do a
This process helps determine which diseases are the most likely to be the cause of the patient’s symptoms and which diseases can be excluded from consideration.
In the medical history, doctors ask people about the details of their symptoms, other known health issues, and previous health-related events.
The details of people's symptoms (for example, how long they have been present, whether they are constant or come and go, and what relieves them) are very important. Doctors also ask people about other symptoms they might not have thought to mention. For example, they typically ask people with a cough whether they also have a runny nose and sore throat (which might suggest the cause of the cough is a viral upper respiratory infection).
Various aspects of a person’s life or background can put them at higher risk of certain diseases. For example, people who smoke are at higher risk of developing lung cancer than nonsmokers, people with diabetes are at higher risk of heart disease than people without diabetes, and the risk of certain diseases can vary according to ethnicity. Therefore, the medical history often includes questions about
When thinking about the potential causes of the symptoms, doctors also take into consideration a person’s
After talking to the person, doctors do a physical examination. The presence or absence of physical signs can help further narrow down the list of diseases that might be causing a person’s symptoms.
Doctors check people's heart rate, breathing rate, and blood pressure (called vital signs) and look at their overall appearance for general signs of illness. General signs of illness include appearing weak, tired, pale, and sweaty or having difficulty breathing. Then doctors examine different parts of the body, usually beginning at the head and working their way down to the feet. Doctors look most carefully at the part of the person's body where their symptoms are located but also check other areas for abnormalities.
Medical tests are done for different reasons, including
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 take the following into consideration:
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
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) testing
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 the next best test.
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).
Screening tests are used to try to detect a disease in people who do not have symptoms. For example, most doctors recommend that all people over 50 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 often 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
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.
Tests are not perfect (see Does Everyone Need a Test?). Results are sometimes normal in people who have the disease being tested for. That is, tests can have false-negative results. Results are sometimes abnormal in people who do not have the disease being tested for. That is, tests can have false-positive results.
Two very important characteristics of a diagnostic test are its sensitivity and its specificity:
Both of these important characteristics can be determined only in well-designed studies.
Therefore, doctors must rely on what is known about the test's accuracy, their knowledge of medicine, and a person's circumstances when interpreting the meaning of a positive or negative test result. When doctors strongly suspect that a certain disease is present—even when an initial test result is negative—they may order additional tests to detect that disease.