The key to surviving all types of cancer is early detection and prompt treatment - before the malignancy spreads and endangers other parts of the body. Colorectal cancer (the most deadly after lung cancer) can be spotted early - sometimes before it starts - by use of a simple home screening device called the stool blood test (SBT).

Some 80 million American men and women are at average-to-high risk of colorectal cancer - most of them only because they are over 40. And according to the American Cancer Society, 130,000 of them will develop the intestinal malignancy this year alone. But with early detection by means of the SBT and other diagnostic tests, "the survival rate can be increased from the current 44 percent to 75 percent," says Dr. Sidney Winawer, chief of gastroenterology at the Memorial Sloan-Kettering Cancer Center in New York. This means, the doctor adds, that 100,000 lives could be saved in the first year. Dr. Winawer advises everyone over 40 to have the SBT, or another colorectal test, annually. The ACS believes, however, that most people can wait until they reach 50 to start such tests.

The SBT has been available from doctors for more than a decade, but many still fail to dispense it to their patients. Now people who are concerned about colorectal cancer don’t have to see a doctor first. They can test themselves at home with one of the nonprescription kits (Detecatest, Colo-trak and Hemoccult Home Test) sold for around $5.00 each in drugstores. A fourth, Early Detector, that’s supposed to be less messy to use, was introduced in the spring of 1986.

Although the SBT is easy, inexpensive and painless, it does have drawbacks. Users must forego red meat, broccoli and certain other foods and medicines for several days to avoid confusing the results. It also necessitates the collection of small specimens of stool, which some people find unpleasant.

Visible blood in the stool may signal the presence of cancer or other disease; anyone finding such blood should se a doctor as soon as possible. The SBT is designed to detect invisible (or occult) blood that may be coming from a precancerous benign polyp in either the colon or the rectum (the upper and lower sections of the large intestine).

Tests provided by doctors usually require that specimens be sent to a laboratory for analysis. But the home kits contain chemicals that allow immediate evaluation by the user. If the test is "positive" - that is, if traces of blood are found in the stool - the next step is to visit a doctor. He or she will undoubtedly refer the patient to a gastroenterologist for a careful examination of the rectum and colon. If the doctor spots a polyp, often he can cut it off at once. Removing a benign polyp almost always guarantees that no cancer will grow at its site. Left untreated, however, about one out of five becomes malignant.

These examinations, which do not require general anesthesia, are usually performed in the doctor’s office - not in an operating room. Special X rays may be needed, however, to explore the upper part of the colon.

Surgery is still the only proven treatment for colorectal cancer. Between 80 and 90 percent of patients survive at least five years if their malignancies are detected and removed while still small, says Dr. LaSalle D. Leffall, Jr., chief of surgery at Howard University in Washington, D.C. But, the surgeon adds, if the cancer has spread to the lymph nodes - from which its cells may circulate throughout the body - the chances of living five years are reduced to fifty-fifty.

When a long segment of the colon must be removed to ensure that all cancer cells are eliminated, the surgeon may not be able to rejoin the two remaining sections. In that case, he must bring the cut end out through an opening, or colostomy, in the abdomen. Body wastes are then voided through this opening and collected in a disposable bag - an inconvenience many people want to avoid.

The good news, says Dr. Leffall, is that only 10 percent of today’s colo-rectal cancer patients have a permanent colostomy. And improved surgical and rehabilitation methods allow most who do have one to lead essentially normal lives.

Treatments are improving. But doctors would rather reduce the need for cancer surgery and rehabilitation by finding and removing polyps before they become malignant, whenever possible. The SBT is the key to this early detection. Two major long-term studies (by the doctors at Sloan-Kettering and the University of Minnesota) are now confirming the test’s value. Dr. Winawer hopes these results, scheduled to be published soon, will remind all physicians of the SBT’s lifesaving potential and persuade them to offer the test routinely to their patients.

Meanwhile, anyone over forty should have an annual SBT - either by getting it from a doctor (as most physicians recommend) or by going to the drugstore for one of the nonprescription tests to use at home.