Evaluating Screening Tests

Evaluating Screening Tests

Screening tests are often used in clinical practice to assess the likelihood that a person has a particular medical condition. The rationale is that, if disease is identified early (before the manifestation of symptoms), then earlier treatment may lead to cure or improved survival or quality of life. This topic is also addressed in the core course in epidemiology in the learning module on Screening for Disease, in which one of the points that is stressed is that screening tests do not necessarily extend life or improve outcomes. In fact, many screening tests have potential adverse effects that need to be considered and weighed against the potential benefits. In addition, one needs to consider other factors when evaluating screening tests, such as their cost, availability, and discomfort.

Screening tests are often laboratory tests that detect particular markers of a specific disease. For example, the prostate-specific antigen (PSA) test for prostate cancer, which measures blood concentrations of PSA, a protein produced by the prostate gland. Many medical evaluations and tests may be thought of as screening procedures as well. For example, blood pressure tests, routine EKGs, breast exams, digital rectal exams, mammograms, routine blood and urine tests, or even questionnaires about behaviors and risk factors might all be considered screening tests. However, it is important to point out that none of these are definitive; they raise a heightened suspicion of disease, but they aren’t diagnostic. A definitive diagnosis generally requires more extensive, sometimes invasive, and more reliable evaluations.

Nevertheless, let’s return to the PSA test as an example of a screening test. In the absence of disease, levels of PSA are low, but elevated PSA levels can occur in the presence of prostate cancer, benign prostatic enlargement (a common condition in older men), and in the presence of infection or inflammation of the prostate gland. Thus, elevated levels of PSA may help identify men with prostate cancer, but they do not provide a definitive diagnosis, which requires biopsies of the prostate gland, in which tissue is sampled by a surgical procedure or by inserting a needle into the gland. The biopsy is then examined by a pathologist under a microscope, and based on the appearance of cells in the biopsy, a judgment is made as to whether the patient has prostate cancer or not. Obviously, if the screening test is to be useful clinically two conditions must be met. First, the test has to provide an advantage in distinguishing between, for example, men with and without prostate cancer. Second, one needs to demonstrate that early identification and treatment of the disease results in some improvement: a decreased probability of dying of the disease, or increased survival, or some measurable improvement in outcome.

One can collect data to examine the ability of a screening procedure to identify individuals with a disease. Suppose that a population of N=120 men over 50 years of age who are considered at high risk for prostate cancer have both the PSA screening test and a biopsy. The PSA results are reported as low, slightly to moderately elevated or highly elevated based on the following levels of measured protein, respectively: 0-2.5, 2.6-19.9 and 20 or more nanograms per milliliter.9 The biopsy results of the study are shown below.

  • The probability that a man has prostate cancer given he has a low level of PSA is P(Prostate Cancer | Low PSA) = 3/64 = 0.047.
  • The probability that a man has prostate cancer given he has a slightly to moderately elevated level of PSA is P(Prostate Cancer | Slightly to Moderately Elevated PSA) = 13/41 = 0.317.
  • The probability that a man has prostate cancer given he has a highly elevated level of PSA is P(Prostate Cancer | Highly Elevated PSA) = 12/15 = 0.80.

Thus, the probability or likelihood that a man has prostate cancer is related to his PSA level.