The Utility of Screening
The Detectable Pre-Clinical Phase
Without screening, diagnosis of disease only occurs after symptoms develop. However, disease frequently begins long before symptoms occur, and even in the absence of symptoms there may be a point at which the disease could be detected by a screening test. The time interval between possible detection by screening and later detection after symptoms is the "detectable pre-clinical phase" or DPCP. We hope that detection of disease in the DPCP will lead to earlier treatment and that this, in turn, will lead to a better outcome. However, this is not always the case.
When Screening Can Be Beneficial
- The disease is serious (e.g., cervical cancer).
- When treatment before symptoms occur is more effective than treatment that is delayed until symptoms appear.
- When the prevalence of disease in the DPCP is relatively high.
By these criteria, blood pressure screening to detect and treat hypertension is an ideal circumstance for screening.
However,screening is not always appropriate:
- Many people with gallstones remain asymptomatic and do not require surgery. If they become symptomatic, the gallbladder can be removed, and the delayed treatment generally causes no problem.
- Even when lung cancer is detected by screening, earlier treatment does not seem to prolong survival substantially.
- Screening is very inefficient when the prevalence of the condition is low.
- There is controversy about the role of PSA (prostate-specific antigen) testing to identify prostate cancer. See the videos below:
Characteristics of a Good Screening Test
- Inexpensive
- Easy to administer
- Minimal discomfort
- Reliable (consistent)
- Valid (distinguishes diseased & non-diseased people)
Sources of Unreliability (Inconsistency)
If a test is reliable, it gives consistent results with repeated tests. Variability in the measurement can be the result of physiologic variation or the result of variables related to the method of testing. For example, if one were using a sphygmomanometer
to measure blood pressure repeatedly over time in a single individual, the results might vary depending on:
- Biological variability (BP normally varies within an individual).
- Instrument variability (is the sphygmomanometer reliable).
- Intra-observer variability (does a given tester perform the test the same way each time).
- Inter-observer variability (do different testers perform the test the same way each time).
The reliability of all tests can potential be affected by one or more of these factors.