Cancer is checked through a combination of screening tests, imaging scans, blood work, and biopsies, depending on the type of cancer and whether you have symptoms. For people without symptoms, routine screening catches cancers early, when they’re most treatable. For people who notice something unusual, the process typically starts with a physical exam and moves through increasingly specific tests until a doctor can confirm or rule out cancer.
Symptoms Worth Getting Checked
Sometimes the first step in checking for cancer is recognizing that something in your body has changed. Not every symptom means cancer, but certain patterns are worth bringing to a doctor. Unexplained bleeding is one of the most important: coughing up blood, blood in your stool (which can look dark or tarry), blood in urine, abnormal vaginal bleeding, or bloody nipple discharge. Lumps or masses you can feel under the skin, especially ones that are new or growing, also warrant evaluation.
Other signals are subtler. Shortness of breath during activities that never winded you before. Difficulty swallowing that gets worse over time. Feeling full after eating very little. Sudden changes in bowel or bladder habits, like new constipation, diarrhea, or difficulty passing urine. None of these guarantee cancer, but they all justify a visit to your doctor, who can figure out the cause.
Checking Your Own Skin
Skin cancer is one of the few cancers you can spot yourself. The standard method uses what’s called the ABCDE rule for evaluating moles and skin spots:
- Asymmetry: one half of the mole doesn’t match the other.
- Border: the edges are ragged, notched, or blurred rather than smooth.
- Color: the color is uneven, with mixed shades of brown, black, tan, or patches of white, red, pink, or blue.
- Diameter: the spot is larger than about 6 millimeters (roughly the size of a pencil eraser), or it’s growing.
- Evolving: the mole has changed in size, shape, or color over the past few weeks or months.
Any mole that hits one or more of these criteria is worth having a dermatologist examine. Regular self-checks, where you look over your entire body in a mirror every few months, make it easier to notice changes early.
Routine Screening by Cancer Type
Screening tests look for cancer before you have any symptoms. The specific tests recommended for you depend on your age, sex, and risk factors.
Breast Cancer
Women aged 40 to 74 are recommended to get a screening mammogram every two years. Women with a strong family history or known genetic mutations may start earlier or screen more frequently.
Cervical Cancer
Women aged 21 to 29 should get a Pap test (cervical cytology) every three years. Starting at age 30 through 65, the options expand: a Pap test every three years, an HPV test every five years, or both tests together every five years.
Colorectal Cancer
Screening now starts at age 45 for people at average risk, a change from the previous recommendation of 50. Screening continues through age 75. Options include stool-based tests you can do at home and colonoscopy, which lets a doctor examine the colon directly and remove precancerous growths on the spot.
Lung Cancer
Annual low-dose CT scans are recommended for adults aged 50 to 80 who have a 20 pack-year smoking history (for example, one pack a day for 20 years) and either still smoke or quit within the past 15 years. Once you’ve been smoke-free for 15 years, screening can stop.
Prostate Cancer
Prostate screening uses a PSA blood test, which measures a protein produced by the prostate gland. Elevated levels can signal cancer, but they can also result from non-cancerous conditions. Screening decisions for prostate cancer are typically made through a conversation between a man and his doctor, weighing individual risk factors.
Blood Tests and Tumor Markers
Blood tests alone can’t diagnose most cancers, but they provide clues. Tumor markers are substances that certain cancers release into the bloodstream. PSA is one of the most well-known. Others include CA-125, which is associated with ovarian cancer, and alpha-fetoprotein (AFP), which can be elevated in liver cancer and certain other tumors.
These markers are more commonly used to monitor how a known cancer is responding to treatment or to watch for recurrence than to make a first diagnosis. The reason is that marker levels can be elevated for non-cancerous reasons, and some cancers don’t produce measurable markers at all. A complete blood count and basic metabolic panel can also reveal indirect signs of cancer, like unexplained anemia or abnormal organ function, that prompt further investigation.
Imaging Tests
When a doctor suspects cancer or needs to learn more about an abnormality, imaging is usually the next step. Each type of scan has different strengths.
CT scans create three-dimensional cross-sections of the body, almost like looking at slices of bread. They can reveal whether a tumor is present and roughly how deep it sits inside the body. CT is one of the most commonly used tools for detecting and evaluating tumors in the chest, abdomen, and pelvis.
MRI uses magnetic fields instead of radiation and is sometimes more sensitive than CT for distinguishing between different types of soft tissue. It’s particularly useful for brain tumors, spinal cord tumors, and cancers in areas where tissue contrast matters.
PET scans work differently. They detect chemical activity rather than structure, highlighting areas where cells are consuming sugar at an abnormally high rate, which cancer cells tend to do. PET scans are more accurate for larger, more aggressive tumors and less reliable for tumors smaller than about 8 millimeters (roughly the size of a pinky nail). They’re especially useful for staging cancer that has come back and for checking whether treatment is working, since dying tumor cells use less sugar over time. PET can sometimes detect cancer when other imaging looks normal.
Ultrasound uses sound waves and is helpful for examining certain areas, like the thyroid, breast, or liver. It can also guide a needle during a biopsy. However, it has limitations for deeper structures and isn’t reliable for scanning the lungs, brain, or large areas of the abdomen and pelvis.
Biopsy: The Definitive Test
A biopsy is often the only way to confirm cancer. It involves removing a small sample of suspicious tissue so a pathologist can examine the cells under a microscope. There are several types, and which one you get depends on where the suspicious area is and how accessible it is.
Fine-needle aspiration uses a thin needle and syringe to draw out fluid and cells. It’s quick and minimally invasive, commonly used for lumps in the thyroid, breast, or lymph nodes. Core needle biopsy uses a slightly larger needle with a cutting tip to extract a small column of tissue, giving the pathologist more material to work with and a better picture of the tissue’s structure.
Surgical biopsy is reserved for situations where the suspicious area can’t be reached with a needle or when needle biopsies have returned inconclusive results. A surgeon makes an incision to access the area and may remove part of the suspicious tissue or all of it. Biopsy results are typically posted to your patient portal, sometimes before your doctor has had a chance to discuss them with you, so it helps to be prepared for that possibility.
Genetic Testing for Cancer Risk
Genetic testing doesn’t check for active cancer. Instead, it looks for inherited mutations that significantly raise your lifetime risk of developing certain cancers. The most well-known are mutations in the BRCA1 and BRCA2 genes, which dramatically increase the risk of breast and ovarian cancer. Mutations in the P53 gene are linked to a wide range of cancer types.
In 2023, the FDA approved a test that can screen for the most common hereditary cancer mutations across 47 genes using a single blood sample. Genetic testing is most useful if you have a strong family history of cancer, particularly if multiple relatives were diagnosed at young ages. A positive result doesn’t mean you’ll get cancer, but it can change your screening schedule and the preventive options available to you.
Liquid Biopsy: A Newer Approach
Liquid biopsy is a blood test that analyzes fragments of tumor DNA circulating in the bloodstream. It’s less invasive than a traditional biopsy and has several potential uses: detecting cancer early, identifying specific genetic features of a tumor to guide treatment, and monitoring whether treatment is working.
One of the most promising applications is tracking treatment response in real time. Falling levels of circulating tumor DNA correlate with a tumor shrinking, while rising levels can signal that the cancer is progressing or developing resistance to treatment. In some cases, these blood-level changes show up before a scan would reveal the same information, creating a window for earlier intervention.
Multi-cancer early detection tests, which analyze DNA methylation patterns in a blood sample, aim to catch multiple cancer types at once and even identify where in the body the cancer likely originated. However, liquid biopsy is still not widely used in routine clinical practice. Most applications remain in clinical trials and require further validation before becoming standard care.
How the Diagnostic Process Unfolds
There is no single test that diagnoses cancer. The process is layered. It typically begins with your doctor asking about your symptoms, personal health history, and family history, followed by a physical exam. From there, they may order lab tests, imaging, or both. If those results raise concern, a biopsy usually follows. The biopsy provides the definitive answer.
Along the way, each test narrows the possibilities. A screening mammogram might find a suspicious spot, an ultrasound might characterize it further, and a core needle biopsy might confirm or rule out cancer. If cancer is confirmed, additional imaging (often a PET or CT scan) helps determine whether it has spread and what stage it’s in. That staging information shapes the treatment plan. The entire process, from first suspicion to confirmed diagnosis, can take anywhere from a few days to several weeks depending on the tests involved and how quickly results come back.