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.
Choose how you want to read this
The whole article, with every detail and caution.
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 work 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. 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. An effective screening test does three things. It finds cancer early. It reduces the chance that someone screened regularly will die from that cancer. And its benefits outweigh its harms. Expert groups recommend mammography for breast cancer, HPV and Pap tests for cervical cancer, colonoscopy and stool tests for colorectal cancer, and low-dose CT for people at higher lung cancer risk from smoking.
What happens after an abnormal screen
Screening is a first step, not a final answer. If a screening result is abnormal, more tests are usually done to find out what it means. Not every abnormal finding is cancer; for example, NCI says most people called back after a mammogram are not found to have breast cancer. That is part of why screening tests can have false-positive results. A false positive looks abnormal even though no cancer is present. Those results may need monitoring or further procedures to rule cancer out.
Same tool, different purpose
Sometimes the same tool shows up in both roles. A scan or exam might screen a person who feels well. The same test might 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, and false-positive or false-negative results. Another is overdiagnosis and overtreatment: finding and treating cancers that would never have caused problems. Because of that balance, whether and when to have a screening test is worth talking over with your doctor. The answer depends on your own risk, values, and preferences.
The bottom line
Screening looks for hidden cancer in people who feel well. Diagnostic tests work 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.
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.
Questions to ask your doctor
Being prepared helps you get the most out of your appointments. Save or print these questions.
Tap a question to save it to your list (kept on this device).
Your next step
Plain-language definitions for both sides of the comparison.
Talk with a trained specialist or navigator
Cancer Explained provides educational guidance, but does not replace trained specialists, social workers, or your medical team.
Talk to a trained cancer information specialist
Free, confidential assistance from NCI Cancer Information Service via phone, chat, or email.
Contact your oncology team
Locate after-hours contact numbers, portal messages, or urgent triage phone lines.
Find a patient navigator
Get one-on-one help with appointments, logistics, translation, and care coordination.
Find a genetic counselor
Discuss inherited mutation risk, family history, and genetic testing options.
Find an oncology social worker
Access emotional counseling, family support groups, and mental health resources.
Find a financial navigator
Locate copay assistance foundations, grant programs, and lodging/travel support.
Find a clinical-trial specialist
Search matching studies and speak with NCI trial information specialists.
Get urgent help
Fever of 100.4°F (38°C) or higher during chemo, severe pain, or trouble breathing: call your cancer care team right away, day or night. Call 911 for an emergency.
Plain-language explanation of the published sources cited on this page. AI-assisted, source-checked, not clinician-reviewed.
Last updated: 2026-10-16Next planned review: 2027-01-14
Review details and limits
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status — Source checked. This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
General education — varies by person. Answers genuinely differ between people. This page explains what commonly varies and points you to your care team for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
Words to know
Tap any term to see what it means.
Knowledge check
0 of 5 answered
This self-assessment checks understanding of educational content only. It is not medical advice.
How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
Read more about our editorial process, our use of AI, and our corrections policy.
Spotted a problem? Report an error — a factual mistake, broken or outdated source, confusing wording, or anything that seems unsafe. Please do not include names, medical record numbers, dates of birth, addresses, or other identifying medical information in your report.
After using this page, do you understand what to do next?
Anonymous — we only record the answer, never who gave it.