Stomach cancer is diagnosed primarily through an upper endoscopy, where a doctor examines the inside of your stomach with a small camera and takes tissue samples for analysis. There is no routine screening program for stomach cancer in the United States for people at average risk, so testing usually begins when you have symptoms or fall into a higher-risk category. The process typically involves several steps, from initial evaluation to imaging, depending on what the first tests reveal.
Upper Endoscopy: The Primary Test
An upper endoscopy (also called an EGD or gastroscopy) is the central test for detecting stomach cancer. During the procedure, a doctor passes a thin, flexible tube with a camera on the end through your mouth, down your esophagus, and into your stomach. You’re given sedation, so you’re asleep and won’t feel pain. The whole thing takes about 10 to 15 minutes.
As the camera moves through the stomach, the doctor closely inspects the lining for anything abnormal. Early stomach cancer doesn’t typically look like a large, obvious mass. According to specialists at Johns Hopkins, what they often see instead are very small, very subtle lesions that require careful inspection. If anything looks suspicious, the doctor uses a tool built into the scope to snip a tiny tissue sample, called a biopsy, right then and there.
Preparing for the Procedure
You’ll need to stop eating solid food at least eight hours before the endoscopy and stop drinking liquids four hours before. This ensures your stomach is empty so the doctor has a clear view. If you take blood-thinning medications, your doctor will likely ask you to pause them in the days leading up to the procedure to reduce any bleeding risk. People with diabetes, heart disease, or high blood pressure will get tailored instructions about their medications.
What Happens After
Recovery from the endoscopy itself is quick. You may have a mild sore throat or feel groggy from the sedation for the rest of the day. If biopsies were taken, you should expect results within about two weeks. A pathologist examines the tissue under a microscope, looking for cancer cells and, if cancer is found, identifying the specific type.
Biopsy and Biomarker Testing
The biopsy is what actually confirms or rules out stomach cancer. A visual inspection during endoscopy can raise suspicion, but only the lab analysis of tissue provides a definitive answer. If cancer cells are present, the pathology team runs additional biomarker tests on the sample to guide treatment decisions. These tests look for specific characteristics of the cancer cells, such as whether they produce unusually high amounts of certain proteins (like HER2 or PD-L1), whether they have a high number of gene mutations, or whether they have defects in their DNA repair systems. These details help oncologists determine which therapies are most likely to work.
Imaging Tests for Staging
If a biopsy confirms stomach cancer, the next step is figuring out how far it has spread. This is called staging, and it relies heavily on imaging.
A CT scan is the workhorse here. It shows the tumor’s location, how deeply it has grown into the stomach wall, whether nearby lymph nodes are involved, and whether cancer has spread to other organs in the abdomen. CT scans are about 86% accurate for determining lymph node involvement, though accuracy drops for very early-stage tumors.
PET/CT scans combine metabolic imaging with structural imaging. They’re particularly useful for detecting cancer that has spread to distant parts of the body. For tumors larger than 3 centimeters, PET/CT picks them up about 97% of the time. But for smaller tumors (under 3 cm), sensitivity drops sharply to around 33%. PET/CT also struggles with early lymph node involvement, partly because cancerous lymph nodes smaller than 3 millimeters are hard to distinguish, and the signal from the main tumor can obscure nearby nodes.
Endoscopic Ultrasound
An endoscopic ultrasound (EUS) is a specialized test that combines endoscopy with ultrasound to get a detailed look at the layers of the stomach wall. The stomach wall has five distinct layers, and the depth to which a tumor has penetrated these layers determines its stage. EUS is especially useful for early-stage cancers, where a doctor needs to know whether the tumor is confined to the inner lining or has started invading deeper muscle layers. This information directly affects whether the cancer can be treated with a less invasive approach or requires surgery.
Blood Tests and Their Limits
Standard blood tests cannot reliably detect stomach cancer, especially in its early stages. Traditional tumor markers used in gastrointestinal cancers, such as CEA, CA 72-4, and CA 19-9, often come back completely normal in people with early-stage stomach cancer. These markers are sometimes used to monitor treatment response or watch for recurrence after diagnosis, but they are not useful as a screening or initial detection tool. Researchers are working on more sensitive blood-based approaches, but none have replaced endoscopy as the primary diagnostic method.
Testing for H. pylori
Infection with H. pylori bacteria is one of the strongest known risk factors for stomach cancer. While testing for H. pylori isn’t a cancer test itself, it’s a common early step in evaluating stomach symptoms and assessing cancer risk. Two non-invasive options are widely available. The urea breath test has a sensitivity of about 92 to 94%, meaning it catches the infection in the vast majority of cases. The stool antigen test is slightly less sensitive at around 83%. A simple blood antibody test exists too, but it only tells you whether you’ve ever been infected, not whether the infection is currently active. If H. pylori is found, treating the infection can significantly lower your future cancer risk.
Genetic Testing for Hereditary Risk
A small percentage of stomach cancers are hereditary. Genetic testing for a mutation in the CDH1 gene is recommended for people who meet specific criteria, which center on a pattern called hereditary diffuse gastric cancer. You may be referred for genetic testing if you were diagnosed with diffuse-type stomach cancer before age 50, if you have multiple family members with stomach cancer (at least one being the diffuse type), or if diffuse stomach cancer and lobular breast cancer both appear in your family. People of Māori descent diagnosed with diffuse gastric cancer at any age also meet the criteria. If CDH1 testing comes back negative in someone who fits these patterns, testing for a related gene called CTNNA1 is the next step.
Who Should Be Screened
The United States has no routine stomach cancer screening program for people at average risk. Screening with upper endoscopy is generally reserved for higher-risk groups: people with chronic stomach inflammation or pernicious anemia (a condition where the stomach can’t absorb vitamin B12 properly), anyone who has had part of their stomach surgically removed in the past, people with a family history of stomach cancer, those with known genetic syndromes, and immigrants from countries where stomach cancer is significantly more common, such as Japan, South Korea, and parts of Central and South America.
If you don’t fall into one of these groups, testing typically starts only when you develop concerning symptoms like unexplained weight loss, persistent stomach pain, difficulty swallowing, or bloody stools. In that scenario, your doctor will likely begin with an upper endoscopy as the first and most informative step.