A screening test refers to a test that is carried out in asymptomatic populations with the aim of detecting early disease. Several examination methods, laboratory tests, and imaging modalities have been used for screening; but all must meet certain criteria before routine use can be advocated.
Oncologic screening has received considerable attention over the past few decades with the aim of reducing the burden of neoplastic diseases, given the possibility of cure in the early stages of many neoplasms.
In recent years, the incidence of cancer has risen. In 2010, approximately 3 million people were diagnosed with cancer and up to 1 million people succumbed to cancer. It is estimated that 3% to 35% of these deaths could have been averted through the adoption of screening methods.
Adherence to current screening recommendations is still imperfect, with the Institute of Medicine estimating that up to 20% of new cancers and up to 30% of cancer-related deaths may be averted through the routine adoption of proven screening methods.
Screening: General Principles
A screening test refers to a test that is carried out in asymptomatic populations with the aim of detecting early disease. These tests are usually not diagnostic but identify patients who warrant further workup using specific confirmatory testing such as a tissue biopsy with histopathologic examination (example: breast mass).
Several examination methods, laboratory tests, and imaging modalities have been used for screening; but all must meet certain criteria before routine use can be advocated.
- The screening test must detect cancers before the onset of symptoms.
- Treatment initiated on the basis of a positive screening test must improve outcome as compared to standard treatment of symptomatic patients.
- The outcome should reflect a reduction in cause-specific mortality, but in certain settings, reduction of all-cause mortality may be reasonable (eg, in the elderly).
- The test or examination should ideally be noninvasive and cost-effective.
The accuracy of screening tests is usually expressed in terms of sensitivity and specificity. Sensitivity refers to the diagnostic power of a test to detect disease in people who truly have the disease, whereas specificity refers to the ability of a test to identify people without the disease. Screening tests ideally have a high specificity and sensitivity.
Screening studies, intrinsically, are biased, which may amplify the potential benefits of the study. The common biases encountered include the following:
- Lead-time bias: This occurs when a screening test can identify a disease early, but early treatment does not result in improved survival.
- Over-diagnosis or length-time bias: Sensitive screening methods can sometimes identify indolent diseases that progress slowly or at all, thus, having minimal impact on survival. Screening in this setting can result in harm to the patient, as a result of the side effects of unnecessary therapy. Some authors cite this bias in their criticism of the routine use of prostatespecific antigen (PSA) screening for prostate cancer.
- Selection bias: This bias reflects randomization and sample size, example: study subjects may differ significantly from the population at large. For example, volunteers in screening studies may have greater health awareness and other lifestyle factors that produce better outcomes, regardless of the screening test. This may overestimate the benefit of the screening test in question.
To minimize the occurrence of bias, recommendations for screening must be generated from well-designed prospective randomized controlled trials, which measure cause-specific mortality. These studies provide the best estimates of the benefits or harms of a screening modality.
It is not always possible to conduct such large-scale population studies, because of cost and logistics. Hence, observational case-control and cohort studies are sometimes used to develop clinical practice guidelines in settings where the disease burden is significant and randomized clinical trials are unavailable.
Widely accepted cancer screening guidelines include those published by the United States Preventive Services Task Force (USPSTF), American Cancer Society (ACS), National Cancer Institute (NCI), and medical specialty societies.
Screening: Future Directions
The emergence of new techniques in molecular biology, particularly proteomics, may help identify the expression of unique surface proteins associated with asymptomatic cancers. High throughput laser desorption machines can identify many such proteins, and these assays may ultimately characterize specific patterns associated with different types of cancer. Although this technique is promising, it is still in its infancy and has not been used for routine cancer screening.
Results from other studies, including a large study evaluating the use of serum cancer antigen 125 measurements and transvaginal ultrasound for the screening of ovarian cancer, may help identify new screening paradigms.
One of the great challenges of screening has been adherence to current screening recommendations. Overall, it is estimated from patient interviews, that although 70% of women had had a mammogram and Pap smear recently, only 35% to 40% of men and women had an FOBT or colonoscopy. Continued education and patient counseling by physicians and physician extenders have been found to double or triple response rates. Disadvantaged populations, including ethnic minorities and patients without health insurance, may be particularly important targets for education and screening.