Terminology 101: Screening
Screening: Services designed to identify people at high risk of having a condition associated with a modifiable adverse outcome, offered to those who have neither symptoms of nor risk factors for the condition in question
Source: DiCenso, A., Guyatt, G., & Ciliska, D. (2005). Evidence-Based Nursing: A Guide to Clinical Practice. St Louis: Elsevier Mosby
Nurses and other health-care providers use various tests to screen for diseases in which early detection is associated with better outcomes (e.g., colon cancer). The tools and procedures they employ can be as simple as a fecal occult blood test (FOBT) kit or as complex as a colonoscopy. Regardless of their nature, all screening tests must have a certain level of accuracy. So, how does one judge how accurate they are?
Before we can answer this question, we first need to be aware of the four possible results of a screening test: true positive (TP), false positive (FP), true negative (TN) or false negative (FN). A TP result occurs when a test correctly classifies a positive case (i.e., a case in which the disease in question is present) as positive. An FP result occurs when the test erroneously classifies a negative case (i.e., a case in which the disease is not present) as positive. A TN result is one in which a negative case is correctly classified as negative, and a FN result is one in which a positive case is erroneously classified as negative. Given that the FP and FN classifications are erroneous ones, they are referred to as misclassifications.
The accuracy of a particular screening test is measured by how well it performs against the gold-standard test in its ability to properly classify patients as either having or not having the disease in question. The lower the rate of misclassification (i.e., the smaller the number of FP and FN results), the more confident we can be that the screening test is an acceptable alternative to the gold-standard test.
Let us consider the example of screening for colon cancer. Although colonoscopy is considered to be the gold-standard test for this condition, it is an expensive, time consuming, invasive and risky procedure. As an alternative to colonoscopy, one could use the less expensive, simpler and less invasive FOBT. It will be deemed to be an appropriate screening test if it performs well against the gold-standard test in terms of its ability to properly classify patients as either having or not having colon cancer.
Although we would ideally like to use tests that produce no misclassifications at all, it is almost impossible to attain this level of accuracy. Instead, if it isn’t feasible to use the gold-standard test because of cost or other considerations, clinicians strive to use an alternative test that has an accuracy level as close as possible to that of the gold standard. Clinicians may be more willing to accept one type of misclassification than another, depending on the primary purpose of the test. Upcoming columns will explore this point further and will describe some measures that are used to quantify various aspects of the accuracy of screening tests.
NurseONE.ca resources on this topic
- Summerton, N. (2007). Patient-Centred Diagnosis.
- Williams, R., Herman, W., Kinmonth, A.-L., & Wareham, N. J. (Eds.). (2003). The Evidence Base for Diabetes Care.
- Patlak, M., & Levit, L. (2010). Policy Issues in the Development of Personalized Medicine in Oncology: Workshop Summary.