What makes a screening test effective

Substantial resources are consequently required for educational campaigns to encourage women to undergo Pap tests. Pain on compression is reported by a small number of women undergoing mammographic screening. Resources are devoted to radiographer training to address this issue.

This is particularly so of screening. If a patient asks a doctor for help the doctor is obliged to do his or her best to help but … if the doctor initiates a screening program there is a presumption that this must benefit the patient [3] ". It is very difficult to determine the benefit of screening for an individual. The distinction between benefits to the community and to individuals needs to be borne in mind when considering recommendations to participate in organised population screening programs.

Participants in screening programs are ostensibly healthy people, so a program should, at the very least, be able to demonstrate evidence of an overall benefit to the community and a minimum of risk that certain individuals may be disadvantaged by the program [1]. Not only is it important that information on the effectiveness of screening programs be available, it should also be disseminated widely. Regular monitoring and evaluation of screening programs is also vital to ensure that effectiveness is maintained and improved where possible.

It is essential to recognise that an organised population approach to screening, which ultimately achieves a net health benefit to a community, can result in adverse outcomes for some individuals.

There is a risk that people who receive false negative results may experience delays in diagnosis and treatment. Some may develop a false sense of security and ignore warning symptoms.

Increasingly, false negative results can give rise to legal action by people whose cancers appear to have been missed. A false positive result can mean that people without the disease undergo follow-up testing that may be uncomfortable, expensive, and, in some cases, potentially harmful. Rarely, this can lead to unnecessary treatment. There may be psychological consequences such as anxiety for both the patient and their family.

For example, a woman with a false positive mammogram undergoing surgical investigation e. A person undergoing a colonoscopy as a result of a false positive faecal occult blood test faces the possibility of a bowel perforation during the procedure.

This risk might be as high as one in [4]. There are concerns that false negative results can give rise to legal action by people whose cancers appear to have been missed.

This must be communicated effectively to the potential participants in a screening program to allow informed consideration of their involvement before any test is done.

Communications must address differences in literacy and language competency to ensure that individuals are properly informed. Implementation of possible screening programs will be influenced by consideration of the equal distribution of limited resources across the whole community for maximum benefit. Resources allocated to a screening program will lower resources available for other health needs. Determining the costs of screening involves the costs of the test and subsequent diagnostic tests and the costs associated with any hazard of the test as well as the costs of over-treatment.

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Genet Test Mol Biomarkers. Cost-effectiveness of additional blood screening tests in the Netherlands. Download references. All data generated or analyzed during this study are included in this published article [and its supplementary information files]. Both authors contributed to i the conception of the review, ii data extraction and analysis, and iii manuscript drafting and revising. The authors agree to be accountable for all aspects of this work. Both authors approve this version to be published.

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. You can also search for this author in PubMed Google Scholar. Correspondence to Eldon Spackman. Reprints and Permissions. Iragorri, N. Assessing the value of screening tools: reviewing the challenges and opportunities of cost-effectiveness analysis. Public Health Rev 39, 17 Download citation. If this orientation is used consistently, the focus for predictive value is on what is going on within each row in the 2 x 2 table, as you will see below.

If a test subject has an abnormal screening test i. In the example we have been using there were 1, subjects whose screening test was positive, but only of these actually had the disease, according to the gold standard diagnosis.

Interpretation: Among those who had a positive screening test, the probability of disease was Negative predictive value: If a test subject has a negative screening test, what is the probability that the subject really does not have the disease?

In the same example, there were 63, subjects whose screening test was negative, and 63, of these were, in fact, free of disease. Interpretation: Among those who had a negative screening test, the probability of being disease-free was This widget will compute sensitivity, specificity, and positive and negative predictive value for you.

Just enter the results of a screening evaluation into the turquoise cells. In the video below, he discusses predictive value. One factor that influences the feasibility of a screening program is the yield , i. This can be estimated from the positive predictive value. Sensitivity and specificity are characteristics of the test and are only influenced by the test characteristics and the criterion of positivity that is selected.

In contrast, the positive predictive value of a test, or the yield, is very dependent on the prevalence of the disease in the population being tested. The higher the prevalence of disease is in the population being screened, the higher the positive predictive values and the yield. Consequently, the primary means of increasing the yield of a screening program is to target the test to groups of people who are at higher risk of developing the disease. To illustrate the effect of prevalence on positive predictive value, consider the yield that would be obtained for HIV testing in three different settings.

The examples below show how drastically the predicative value varies among three groups of test subjects. All three show the effects of screening , subjects.

The only thing that is different among these three populations is the prevalence of previously undiagnosed HIV. The 1 st scenario illustrates the yield if the screening program were conducted in female blood donors, in whom the prevalence of disease is only 0. What these three scenarios illustrate is that if you have limited resources for screening, and you want to get the most "bang for the buck," target a subset of the population that is likely to have a higher prevalence of disease, and don't screen subsets who are very unlikely to be diseased.

Diagnostic measures included the area under the receiver-operating characteristic curve, sensitivity, specificity, and likelihood ratios. The area under the ROC curve was 0. Question: What was the positive predictive value in this study? Hint: You have to use the information provided to piece together the complete 2x2 table; then compute the PPV. See if you can do this before looking at the answer. In the video below, he discusses serial and parallel diagnostic testing.

At first glance screening would seem to be a good thing to do, but there are consequences to screening that carry a cost, and the potential benefits of screening need to be weighed against the risks, especially in subsets of the population that have low prevalence of disease! Specifically, one needs to consider what happens to the people who had a positive screening test but turned out not to have the disease false positives.

Women between years old can get breast cancer, but the probability is extremely low and the sensitivity of mammography is low because younger women have denser breast tissue.

Not only will the yield be low, but many of the false positives will be subjected to extreme anxiety and worry. They may also undergo invasive diagnostic tests such as needle biopsy and surgical biopsy unnecessarily. In the case of fecal blood testing for colorectal cancer, patients with positive screening tests will undergo colonoscopy, which is expensive, inconvenient, and uncomfortable, and it carries its own risks such as accidental perforation of the colon.

Such complications are uncommon, but they do occur. The other problem is false negatives , who will be reassured that they don't have disease, when they really do. These hazards of screening must be considered before a screening program is undertaken. For a very relevant look at this, see the following brief article from the New York Times on the potential harms of screening for prostate cancer.

Link to the article. There is concern among some that there is an inordinate emphasis on early diagnosis of disease and that the increasingly aggressive pursuit of abnormalities among people without symptoms is leading to actually harm and great cost without reaping any benefits. Even if a test accurately and efficiently identifies people with pre-clinical disease, its effectiveness is ultimately measured by its ability to reduce morbidity and mortality of the disease.

The most definitive measure of efficacy is the difference in cause-specific mortality between those diagnosed by screening versus those diagnosed by symptoms. There are several study designs which can potentially be used to evaluate the efficacy of screening.

These include correlational studies that examine trends in disease-specific mortality over time, correlating them with the frequency of screening in a population. However,1 these are measures for entire populations, and cannot establish that decreased mortality is occurring among those being screened; 2 one cannot adjust for confounding; and 3 one cannot determine optimal screening strategies for subsets of the population.

Case-control and cohort studies are frequently used to evaluate screening, but their chief limitation is that the study groups may not be comparable because of confounders, volunteer bias, lead-time bias, and length-time bias. Because of these limitations, the optimal means of evaluating efficacy of a screening program is to conduct a randomized clinical trial RCT with a large enough sample to ensure control of potential confounding factors. However, the costs and ethical problems associated with RCTs for screening can be substantial, and much data will continue to come from observational studies.

Screening programs also tend to look better than they really are because of several factors:. People who choose to participate in screening programs tend to be healthier, have healthier lifestyles, and they tend to adhere to therapy better, and their outcomes tend to be better because of this.

However, volunteers may also represent the "worried well," i. All of these factors can bias the apparent benefit of screening. The premise of screening is that it allows you to identify disease earlier, so you can initiate treatment at an early stage in order to effect cure or at least longer survival. Screening can give you a jump on the disease; this "lead-time" is a good thing, but it can bias the efficacy of screening.