Medical Testing Decisions

ByMichael Joseph Pistoria, MEng, DO, Lehigh Valley Hospital - Coordinated Health
Reviewed/Revised Aug 2021 | Modified Jan 2023

Because many different diseases can cause the same symptoms, it can be challenging for doctors and other primary care practitioners 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 and The Science of Medicine.)

Gathering Basic Medical Information

Doctors do a

  • Medical history

  • Physical examination

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.

Medical history

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

  • Current and past illnesses

  • Regular medications

  • Diseases that run in the family

  • History of travel overseas

  • Sexual practices

  • Use of cigarettes, alcohol, and/or recreational drugs

  • Occupation and hobbies

When thinking about the potential causes of the symptoms, doctors also take into consideration a person’s

  • Age

  • Gender

  • Ethnicity

Physical examination

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.

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

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) testing

  • People with certain types of artificial heart pacemaker 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 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).

Does Everyone Need a Test?

In short, no. Although many people find medical tests reassuring, test results are not always right:

  • Results are sometimes normal in people who have a disease (false negative).

  • Results are sometimes abnormal in people who do not have 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 people did have a UTI, that means that in 100 little girls like this one

  • The 5 who actually had a UTI would have a truly positive test.


  • 10 would have a falsely positive test.

In other words, in this 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

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

Did You Know...

  • 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.

Classification tests

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.

Monitoring tests

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.

Interpreting Medical Test Results

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:

  • Sensitivity: The likelihood that results will be abnormal in people with the disease being tested for

  • Specificity: The likelihood that results will be normal in people without the disease

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.

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