The short answer
Screening tests aim to find cancer before it causes symptoms, when it may be easier to treat. Diagnostic tests are used to find out what is happening when someone has a symptom or an abnormal screening result. A screening test can suggest something needs a closer look, but it does not by itself confirm a diagnosis.
Screening tests aim to find cancer before it causes symptoms, in people who feel well.
Diagnostic tests are used to figure out what is going on when there is a symptom or an abnormal result.
A screening test can flag something to check further, but it does not confirm a diagnosis on its own.
An abnormal screening result often leads to more tests, and many such findings turn out not to be cancer.
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The full explanation.
The short version
These two kinds of tests have different jobs:
- Screening tests aim to find cancer before it causes symptoms, in people who feel well.
- Diagnostic tests are used to figure out what is going on when there is a symptom or an abnormal screening result.
A screening test can flag that something needs a closer look, but it does not by itself confirm a diagnosis.
What screening is for
Cancer screening tests aim to find cancer early, when it may be easier to treat successfully. An effective screening test finds cancer early, reduces the chance that someone screened regularly will die from the cancer, and has more potential benefits than harms. Examples that expert groups recommend include mammography for breast cancer, HPV and Pap tests for cervical cancer, colonoscopy and stool tests for colorectal cancer, and low-dose CT for lung cancer in people at higher risk because of smoking.
What happens after an abnormal screen
Screening is a first step, not a final answer. If a screening result is abnormal or positive, more tests are usually done to find out what it means. Many of these findings turn out not to be cancer. This is part of why screening tests can have false-positive results — findings that look abnormal even though no cancer is present. Those results may need monitoring or further procedures to rule out cancer.
Same tool, different purpose
Sometimes the same tool shows up in both roles. A scan or exam might be used to screen a person who feels well, or to help diagnose a person who has symptoms or an abnormal result. The key difference is the reason for the test. Your care team knows which purpose applies to your situation.
Benefits and harms
Screening has real benefits, but it also has possible harms. These can include bleeding or other physical damage, false-positive and false-negative results, and overdiagnosis and overtreatment — finding and treating cancers that would not have caused problems and did not need treatment. Because of this balance, whether and when to have a screening test is worth talking over with your doctor, based on your own risk, values, and preferences.
The bottom line
Screening looks for hidden cancer in people who feel well; diagnostic tests figure out what is going on when there is a symptom or an abnormal result. An abnormal screen usually means more tests, not a diagnosis. Ask your care team which screening tests are recommended for you, what a test is for, and what happens if a result is not normal.
Words to know
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Common questions
▸What is the difference between a screening test and a diagnostic test?
A screening test looks for cancer before it causes symptoms, in people who feel well. A diagnostic test is used to find out what is going on when there is a symptom or an abnormal screening result. Screening finds things to check; diagnosis figures out what they are.
▸If a screening test is abnormal, does that mean I have cancer?
Not necessarily. An abnormal or positive screening result often leads to more tests, and many such findings turn out not to be cancer. Screening tests can have false-positive results — findings that look abnormal even though no cancer is present. Your care team explains any next steps.
▸Why do the same tools sometimes appear in both?
Some tools, like certain scans, can be used for screening in people without symptoms or for diagnosis in people who have symptoms or an abnormal result. The difference is in the reason for the test, not always the machine. Your care team knows which purpose applies to you.
▸Does screening have downsides?
Yes. Along with benefits, possible harms of screening include bleeding or other physical damage, false-positive and false-negative results, and overdiagnosis and overtreatment — finding and treating cancers that would not have caused problems. Weighing these is worth a conversation with your doctor.
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