Screening for Cancer: Determining the value of screening tests

Screening tests for other types of cancers continue to be evaluated to determine if early detection results in a cure or increased survival. Four examples of cancer screening tests that are still being studied include (1) CA-125 blood test with transvaginal ultrasound (to look for ovarian cancer in women), (2) chest X-ray (to screen for lung cancer), (3) flexible sigmoidoscopy (to look for colon cancer) and (4) digital rectal exam (DRE) plus blood tests for PSA for prostate cancer screening in men.

While it may seem that screening for cancer would always be beneficial, there are some risks associated with this process. One issue with screening older individuals is that not all detected cancers progress to further medical problems or death. This issue is highlighted in a recent news article on our site, Prostate Cancer Screening Studies Don't Settle Question of Clinical Utility.

Another limitation is that some abnormal results do not always indicate the presence of disease. Tests for screening are designed to offer the highest ability to pick up a disease when it is present (termed sensitivity). As a result, not all abnormal (positive) results truly indicate disease (called "true" positive results); some will be abnormal even in persons who don't have the disease (called false positive results). Screening tests are meant to be followed up with other tests to confirm whether the person actually has the disease or not. (For more on this, see the articles on Reference Ranges & What They Mean and How Reliable is Laboratory Testing?)

A research study called the Prostate, Lung, Colon, and Ovarian (PLCO) Cancer Screening Trial was designed to evaluate the impact of screening for these four major types of cancer relative to the number of deaths due to those cancers. Preliminary results from one part of the study published in the May/June 2009 issue of the Annals of Family Medicine, noted the high rate of false positive results with certain screening tests.

For purposes of the study, the investigators defined a "false-positive" as a positive screening test result that occurred in a person who had no other evidence of cancer for 3 years beyond the test. Diagnostic procedures used to investigate positive screening results and determine if cancer was present ranged from minimally invasive tests, such as colonoscopy, to somewhat more invasive tests, such as a biopsy to major surgery.

According to the researchers, the likelihood of false positive results is cumulative; that is, as the number of screenings tests increases, so does the chance of a result indicating cancer when none exists. They determined that by the fourth screening test, the chance of a false positive result was about 37% for men and about 26% for women.

The researchers also noted that the impact of cancer screening on preventing deaths from prostate, lung, colon, and ovarian cancer will not be known until the conclusion of this trial.

It is important to understand the risks associated with follow-up procedures for a positive screening test when there is no cancer as well as the benefits associated with early detection and treatment of cancer. Balancing these considerations can help patients and their doctors make prudent decisions about which cancer screening tools should be used, in addition to when and how often to test.