During the last year, three friends have been battling colon cancer, but in only one was the disease detected before it had spread to other organs. That one is a physician in his 50's who has had frequent colonoscopies because his mother died of the disease at an early age.

He, on the other hand, can look forward to a long and healthy life, thanks to early detection.

The lesson here is obvious: colorectal cancer, as it is officially called, approaches being 100 percent curable, even preventable, if everyone at risk would take advantage of well-established tests that can find both localized cancer and precancers.

And everyone over 50 is, by definition, at risk, along with younger people with hereditary or other conditions that increase their risk.

The doctor was tested often because he knew that a family history of colon cancer greatly increased his chances of developing it, a fact confirmed by the discovery of several precancerous polyps that had been removed in previous colonoscopies.

You may have seen the American Cancer Society ads urging everyone to ''get the test, get the polyp, get the cure.'' Chances are, if you know you should be tested, you've been putting it off because you've heard the exams are anything but pleasant.

But, as one who has had several of the tests, I can assure you of two things: they are not nearly as discomforting as most people think, and they are far more ''pleasant'' than dealing with advanced cancer.

Curable in Early Stage

Colorectal cancer is exceeded only by cancers of the lung and breast as the most commonly diagnosed life-threatening malignancy in this country, and it is exceeded only by lung cancer as the leading cause of cancer deaths. Colorectal cancer will be found in an estimated 147,500 people in the United States this year, and it will claim the lives of 57,100.

But colon cancer is the most curable of major cancers when treated while still localized. Now, 90 percent of patients whose cancers are discovered before they have spread are alive and well five years later. The rate is 100 percent among those who have precancerous polyps detected and removed.

Colon cancer is so amenable to early detection, cure and prevention because it arises slowly in benign growths called adenomatous polyps that are easily detected and removed.

Most adenomas remain benign; each year only 5 in 2,000 become cancerous, a transformation that occurs on average over 10 years. The larger the adenoma, the more likely it will become malignant. This provides an incredible window of opportunity for early detection and prevention.

Well-known figures who died after getting colon cancer include the actress Audrey Hepburn and the Peanuts cartoonist Charles Schulz. On the other hand, Justice Ruth Bader Ginsburg and President Ronald Reagan both survived it.

Despite the publicity from these cases, fewer than 40 percent of Americans who should be tested undergo any screening tests for colon cancer.

People at especially high risk of developing this disease include those with a personal history of polyps or colon cancer or chronic inflammatory bowel disease (ulcerative colitis or Crohn's disease); those with a parent, sibling or child who had this cancer or polyps before age 60 or two relatives who had them at any age; and those with certain hereditary conditions, like familial adenomatous polyposis, Gardner's syndrome or hereditary nonpolyposis colorectal cancer.

In everyone, risk rises with age.

Taking the Tests

A number of options are available for colon cancer screening, and several improvements are in the works, though one, the colonoscopy, is the gold standard, providing a clear view of the entire bowel and permitting simultaneous removal of suspicious-looking polyps.

Tests for fecal occult blood are the most widely used, though the least accurate. They involve taking smears of three consecutive bowel movements and applying them to a kit that is then analyzed.

To reduce false positive readings, the test requires abstaining from aspirin and other nonsteroidal antiinflammatory drugs for seven days before and during the test and avoiding all red meats, vitamin C and citrus fruits and juices for three days before and during the test.

Of course, cancers don't always bleed, and other tissues, like hemorrhoids, sometimes do, resulting in a mistaken test result. But used annually, these tests can reduce colon cancer deaths by 15 percent to 43 percent, studies show.

Fecal blood tests are more accurate if this annual test is combined with a flexible sigmoidoscopy every five years. The sigmoidoscope examines the lower part of the bowel, where up to 60 percent of the cancers arise and can reduce cancer deaths in this area by 60 percent to 85 percent. It requires a pre-examination bowel cleansing with two enemas.

A third test, a double-contrast barium enema, requires a more thorough bowel cleansing, followed by administration of barium and then air into the bowel, which is then examined by X-ray. The exam is expensive and takes about an hour. It is best at finding larger polyps, which are the ones that are most likely to be troublesome.

It too should be repeated every five years in people with no symptoms or special risk factors. If anything suspicious is noted on the X-ray, the next step is a colonoscopy.

Colonoscopy is the most accurate and most expensive screening exam. In people at average risk, it should be repeated every 10 years starting at age 50. It is the best test for people with special risk factors, who should be tested far more often, sometimes as often as every six months.

This test, performed under sedation, requires a thorough bowel cleansing using an oral solution to assure a clear view of the lining. It examines the entire six feet of colon and rectum using a flexible scope. A device can be threaded through it to remove any suspicious polyps.

It costs $700 to $1,100. I can assure anyone who is hesitant that while the preparation is anything but pleasant, the exam itself is painless. I've yet to feel a thing.

You may have heard advertisements for a seemingly simple ''virtual colonoscopy,'' or colonography, a C.T. scan providing three-dimensional colon images. It requires a 48-hour low-fiber diet and a complete bowel cleansing to be accurate.

The bowel is filled with air and serial X-rays are taken. The test is as expensive as a regular colonoscopy.

There are several disadvantages to this virtual exam. First, if anything suspicious is noted, an invasive colonoscopy will still be needed to remove the tissue.

In about 15 percent of cases, misleading positive readings result in unneeded colonoscopies. And, it is not yet known whether the test can detect relatively rare but more deadly flat adenomas that don't protrude into the opening of the colon.

Photos: Well-known people who had colon cancer include, clockwise from top left, the cartoonist Charles Schulz, Justice Ruth Bader Ginsburg, the actress Audrey Hepburn and former President Ronald Reagan. (Photographs by Associated Press)