THE American College of Physicians and the nation's largest insurer, Blue Cross and Blue Shield, say they have developed guidelines to determine when 15 commonly performed medical tests are unnecessary.

What was particularly striking about their announcement this week was the frequency with which these tests are ordered. A spokesman for the insurer said that about 20 percent of all diagnostic tests were unwarranted.

Blue Cross will use the guidelines to challenge bills submitted for their subscribers by doctors. By using the guidelines, the insurer estimated that the $30 billion Americans spend annually for these tests could be cut by $6 billion.

Why are so many unnecessary tests being performed? One major cause, according to Blue Cross, is doctors who practice so-called defensive medicine. The American Medical Association said that one of the more frequent causes for litigation is a doctor's failure to diagnose a problem. A 1983 survey of doctors by the American Medical Association found that 41 percent said they ordered additional laboratory and diagnostic tests because they feared being sued for failure to notice a problem.

But consumer groups concerned with health issues say that some tests could be avoided if patients simply understood more about the procedures and asked more questions.

Although patients now seem to be taking more of an interest in their treatment, medical experts say they rarely ask questions about medical tests.

''Patients need to understand the tests that are being performed on them and they need to be included in the decision on whether to have them performed,'' said Dr. Marc Manley, a physician with the Public Citizen Health Research Group, a consumer advocacy organization that was founded by Ralph Nader.

Dr. Manley said that patients who understood the purpose of a test could make a better judgement about whether it should be performed. For example, Dr. Manley said that chest X-rays, which are commonly ordered for any patient being admitted to a hospital, were often unnecessary.

''Particularly with X-rays, consumers need to exercise some caution,'' Dr. Manley said. ''No X-ray should be performed without adequate justification because every X-ray involves some small amount of radiation and radiation can have some cumulative health effects.''

The new test guidelines, developed by the American College of Physicians, a group representing 63,000 internists and primary care doctors, also said chest X-rays were ''a prime example of unnecessary testing.'' The tests cost $35 to $80; $2 billion is spent each year.

Michael Jacobson, the executive director of the Center for Science in the Public Interest, said consumers needed to understand medical tests in order to interpret their results. One example, he said, is a test for elevated cholesterol levels in the blood. Doctors will often simply say that the level the test found was ''normal,'' Mr. Jacobson said, but people with what some physicians consider normal cholesterol levels have died from heart disease. ''Normal is not necessarily good,'' he added.

There are several books on medical tests that are available. The third edition of ''The Patient's Guide to Medical Tests,'' (Facts on File Publications, $12.95) by Cathey Pinckney and Dr. Edward R. Pinckney was published this month. ''The People's Book of Medical Tests'' by Dr. David S. Sobel and Dr. Tom Ferguson (Summit Books, $12.95) was published in 1985.

Before agreeing to a test, both books suggest that your doctor should be able to explain how the test results will help diagnose your problem or change the planned treatment. The most obvious source of all information about medical tests is your doctor.

The guidelines cover the following tests: 1. chest X-ray; 2. electrocardiogram; 3. complete blood count; 4. serum electrolytes and serum osmolality (a blood chemistry analysis); 5. biochemical profiles in ambulatory screening and preadmission testing for adults; 6. blood urea nitrogen concentration and serum creatinine (to assess kidney function); 7. urinalysis, urine culture and other tests for the diagnosis of acute infections in women; 8. activated partial thromboplastin time and prothrombic time (to measure blood coagulation); 9. throat cultures for diagnosing group A streptococcal pharyngitis (for strep throat); 10. syphilis tests; 11. arterial blood gas analysis; 12. erythrocyte sedimentation (to measure the rate at which blood cells settle in anticoagulated blood); 13. carcinoembryonic antigen in fetal life (a cancer test); 14. cardiac enzyme assays (to diagnose heart attack), and 15. blood cultures.