Cancer Screening Tests: The Basics
What is a screening test?
Cancer screening tests look for cancer before a person has any symptoms. When abnormal tissue or cancer is found early, it may be easier to treat or cure. By the time symptoms appear, the cancer may be more advanced and could be harder to treat or cure.
Why do a screening test?
Screening tests are used for types of cancer that are relatively common in the population; this means the disease has a high incidence rate. Screening tests are also used for diseases where there are effective treatments available. For conditions where there is no known effective treatment, screening is of limited use. When your doctor suggests a screening test, it does not necessarily mean that he/she believes you have cancer. Screening tests detect the possibility that cancer is present before you have symptoms.
There are different kinds of screening tests.
Screening tests include the following:
- Physical exam: An exam of the body to check general signs of health, including checking for signs of disease. An example is a breast exam done to detect breast lumps or masses.
- Laboratory tests: Samplings of tissue, blood, urine, or other substances in the body. For example, a blood test that measures PSA (prostate specific antigen) is used to screen for prostate cancer
- Imaging procedures: Take pictures of areas inside the body, such as a colonoscopy, the screening test for colon cancer.
What Makes a Good Screening Test?
A good screening test is a safe test.
A physical exam, lab testing, and imaging typically pose very little risk to patients. The more invasive the test, the higher the risk.
A good screening test gives useful results.
This means the test should reliably find disease when it is present in the patient and find no disease if it is not present. . A false-positive test occurs when test results appear to be abnormal even though there is actually no cancer. A false-negative is when test results show no cancer when there really is cancer. No test is perfect: a perfect test would give only true positive and true negative results, but a good screening test should have an acceptably low rate of false-positive and false-negative results. False-positive results can create undue stress and can lead to other unnecessary testing. False-negative results can delay treatment.
The terminology used to describe the usefulness of a test are: sensitivity and specificity, which are described below.
Sensitivity is the "true positive rate." The closer the sensitivity is to 100%, the more likely that a positive result actually means that the patient has a disease. To calculate sensitivity, use the formula a/(a+c), where "a" is the number of true positives and "c" is the number of false negatives.
Specificity is the "true negative rate." The closer the specificity is to 100%, the more likely a negative result means that the patient is truly disease-free. Specificity can be calculated with the formula d/(b+ d) where "d" is the number of true negatives and "b" is number of false positives.
The best screening tests have high sensitivity and high specificity.
Recommended Screening Tests for the General Public (Based on your personal and family history, your doctor may have other recommendations for you.)
As women get older, their chances of getting breast cancer increase. A mammogram is an x-ray test that can find a breast cancer when it is too small to be felt. Most breast cancers have a higher chance of cure if detected and treated early. There is debate over the age to begin screening mammography, however, most recommend a mammogram every 1 to 2 years starting at age 40 or age 50.
Cancer of the Cervix
Sexually active women are at risk for cancer of the cervix. A Pap smear uses a small brush to sample cells of the cervix, which are then examined under a microscope. Pap smears can find cancer and pre-cancerous changes of the cervix early, when there is a higher chance of cure. This test may also be combined with a test for HPV (a virus known to cause cervical cancer) in women over 30 years of age.
Women should have a Pap test starting at age 21, or no later than three years after first sexual intercourse. A Pap smear should be done every 1-2 years; for women over 30 who have had three consecutive normal Pap smears, the test can be spread out to every three years. Women over age 70 who have and have had regular, normal Pap tests should discuss with their doctors how often Pap screening should be done. Women with certain risk factors may need more frequent screening, including those who have HIV, are immunosuppressed, were exposed to diethylstilbestrol (DES) in utero, and have been treated for cervical intraepithelial neoplasia (CIN) 2, CIN 3, or cervical cancer.
Colon cancer screening is recommended starting at age 50.
Fecal Occult Blood Test — Stool samples are taken to test for small amounts of blood in your stool. There are many conditions and medications, which can give a positive result, therefore, in lieu of this test, a doctor may recommend either a sigmoidoscopy or colonoscopy. This test alone is not an effective substitute for sigmoidoscopy or colonoscopy and is best used in combination with these tests.
Sigmoidoscopy or Colonoscopy — These tests look inside the rectum and colon using a small, lighted tube. Biopsies can be taken if there are any abnormal looking areas and potentially precancerous polyps can be removed. These tests are more invasive than fecal occult blood testing, but are more sensitive and specific. They are recommended once every 5 to 10 years, or more frequently if you have a family history of colon cancer and/or personal history of abnormal polyps.
Prostate cancer is more common in men over age 50, in African Americans, and in men with a family history of prostate cancer. Screening tests include a digital rectal exam to feel for nodules on the prostate, and the PSA (prostate-specific antigen) blood test to help detect prostate cancer. There is debate over whether routine screening should be recommended to the general population, and if so, at what age it should start, so discuss this with your doctor.
The Limitations of Screening Tests
Unfortunately, not every type of cancer has a good screening test. A few examples are reviewed below.
Tumor markers, substances secreted into the bloodstream by certain tumors, have a very limited role in cancer screening. In ovarian cancer, many women are found to have an elevated level of the tumor marker CA-125. So, why do we not check CA-125 in all women? CA-125 may not lead to early diagnosis because it is not sensitive or specific for ovarian cancer. It can be expressed in a number of gynecologic (eg, endometrium, fallopian tube) and nongynecologic (eg, pancreas, breast, colon, lung) cancers, as well as in benign conditions such as endometriosis and pregnancy. However, in cases of ovarian cancer where CA-125 is elevated, it can be used clinically after diagnosis to determine response to treatment and to detect relapse. Researchers continue to study the best uses for CA-125.
In lung cancer, there have been screening studies using routine radiography such as chest X-rays or CT scans. Screening with sputum cytology (taking a sample of mucous from the lungs that is examined for cancer cells) has also been done. While these tests can detect lung cancer, they have not been shown to decrease lung cancer death rates. We are still looking to see if there are other screening tests that can be used in lung cancer. For example, screening with helical CT scans as compared to basic x-ray screening is being studied on a clinical trial called the National Lung Screening Trial.
If signs of cancer are detected on a screening test, more definitive and invasive follow up tests are performed to confirm the diagnosis. Controversy arises when it is not clear if the benefits of screening outweigh the risks of follow-up diagnostic tests and cancer treatments. Ultimately, whether or not you have a particular screening test is something you should decide with your doctor after discussing your risk factors and personal preferences.
- Bach PB, Jett JR, Pastorino U, et al. Computed tomography screening and lung cancer outcomes. JAMA; 2007. 297(9):953-961.
- Hogberg T, Kagedal B: Long-term follow-up of ovarian cancer with monthly determinations of serum CA-125. Gynecologic Oncology 46(2): 191-198, 1992.
- The Merck Manual Online Medical Library
- National Cancer Institute
- Nelson HD, Tyne K, Naik A, et al. Screening for breast cancer: an update for the U.S. Preventive Services Task Force. Ann Intern Med 2009;151:727-37.
- U.S. Preventive Services Task Force. Lung Cancer Screening: Recommendation Statement