Stomach cancer is diagnosed primarily through an upper endoscopy, a procedure where a gastroenterologist passes a small camera into your stomach to look for abnormal tissue and collect samples for lab analysis. If cancer is found, imaging tests like CT scans and ultrasounds determine how far it has spread. The full diagnostic process typically involves several steps, from the initial scope to staging and molecular testing that guides treatment decisions.
Upper Endoscopy: The Primary Diagnostic Tool
An upper endoscopy (also called an EGD) is the most direct way to check for stomach cancer. A gastroenterologist threads a thin, flexible tube with a lighted camera on its tip down your throat, through your esophagus, and into your stomach and the upper part of your small intestine. The camera sends live images to a screen, giving the doctor a close-up view of the stomach lining.
Standard white-light endoscopy catches most visible tumors, but smaller or flatter lesions can be harder to spot. Newer techniques using narrow-band imaging, which highlights surface patterns with filtered light, reach accuracy and specificity rates above 90% for detecting early cancers. AI-assisted systems are also improving detection. In one study, an AI tool called ENDOANGEL achieved 93% specificity and 91% accuracy, significantly outperforming the endoscopists it was compared against.
If the doctor sees anything suspicious, they use tiny tools attached to the endoscope to take tissue samples on the spot. This is the biopsy, and it’s the only way to confirm whether cancer cells are present.
What Happens During a Biopsy
During the endoscopy, the gastroenterologist removes small pieces of tissue from multiple areas of the stomach lining. The standard approach, known as the Sydney System protocol, involves collecting five separate samples from different stomach regions. Any visible lesion or abnormal-looking area gets sampled separately. You won’t feel the tissue being removed since the stomach lining lacks the same pain receptors as your skin.
Those samples go to a pathology lab, where a specialist examines the cells under a microscope to determine whether they’re cancerous, precancerous, or benign. Results typically take several days to a week. If cancer is confirmed, the pathologist notes the type of cancer cell and how abnormal the cells look, which helps determine aggressiveness.
Preparing for an Endoscopy
Preparation is straightforward but important. Five days before the procedure, you’ll need to stop taking common anti-inflammatory medications like ibuprofen and naproxen. The night before, don’t eat any solid food after midnight. On the day of the procedure, avoid all food and drink for at least eight hours beforehand. If you take daily medications, you can usually take them four hours before the exam with small sips of water, but skip antacids.
You’ll receive sedation before the scope goes in, so you’ll be relaxed or lightly asleep during the procedure. It generally takes 15 to 30 minutes. Because of the sedation, you’ll need someone to drive you home. Most people feel fine by the next day, though mild throat soreness is common.
Staging Tests After a Diagnosis
Once a biopsy confirms stomach cancer, the next step is figuring out how far it has spread. This is called staging, and it drives every treatment decision that follows.
An endoscopic ultrasound uses sound waves from inside the stomach to show how deeply the tumor has grown into the stomach wall. It can also reveal whether nearby lymph nodes look abnormal. If suspicious lymph nodes are visible on ultrasound, a doctor can guide a fine needle through the scope to collect tissue from them for testing.
CT scans create detailed cross-sectional images of the chest, abdomen, and pelvis to check whether cancer has reached the lymph nodes, liver, lungs, or other organs. PET scans, which detect areas of unusually high metabolic activity, may be used alongside CT to identify smaller areas of spread that a CT alone might miss. Together, these scans build a complete picture of where the cancer is and isn’t, which determines whether surgery is an option and what kind of treatment plan makes sense.
Biomarker Testing on Tumor Tissue
If you’re diagnosed with advanced stomach cancer, your tumor tissue will likely undergo molecular testing to look for specific proteins and genetic features that affect which treatments will work best for you. Two of the most commonly tested markers are HER2, a protein that drives cell growth in some stomach cancers, and PD-L1, a protein that helps cancer cells hide from the immune system.
Tumors that overproduce HER2 can be treated with targeted drugs that block that protein. PD-L1 testing evaluates both the tumor cells and the surrounding immune cells to determine whether immunotherapy is likely to be effective. These tests are run on the same tissue collected during your biopsy or surgery, so they don’t require a separate procedure.
Blood Tests and Liquid Biopsies
Standard blood tests can reveal signs that suggest stomach cancer, like anemia from slow internal bleeding or elevated tumor markers, but they can’t diagnose it on their own. A complete blood count and basic chemistry panel are usually part of the initial workup.
Liquid biopsies, which look for cancer signals circulating in the blood, are an active area of development for stomach cancer. Circulating tumor DNA, which works well for monitoring some other cancers, is too rare in early gastric cancer to serve as a reliable screening tool. Researchers have developed a newer approach that analyzes genetic material carried inside tiny particles shed by tumors into the bloodstream. Early results are promising, but liquid biopsies are not yet part of standard diagnostic practice for stomach cancer.
Who Should Be Screened
Unlike colorectal cancer, there is no universal screening recommendation for stomach cancer in the United States. Screening is targeted at people with elevated risk. Updated 2025 guidelines from the American Gastroenterological Association recommend endoscopic screening for several groups:
- First-generation immigrants from countries with moderate to high rates of stomach cancer (parts of East Asia, Central and South America, Eastern Europe)
- Non-White racial and ethnic groups, who face higher gastric cancer rates in the U.S.
- People with a first-degree relative (parent, sibling, or child) who had stomach cancer
- People with hereditary cancer syndromes linked to gastric cancer
- People with a history of H. pylori infection combined with at least one additional risk factor, such as smoking, a high-salt diet, or persistent poverty
The recommended starting age for endoscopic screening is 45, aligning it with current colorectal cancer screening guidelines. How long screening continues depends on your overall health and other medical conditions.
Separately, the American College of Gastroenterology recommends testing for and treating H. pylori, the bacterial infection that causes most stomach cancers worldwide, in high-risk populations. This includes immigrants from high-prevalence countries, people with precancerous stomach changes like atrophic gastritis or intestinal metaplasia, and those with a family history of gastric cancer. Treating H. pylori before it causes lasting damage to the stomach lining is one of the most effective ways to reduce risk.