Screening of Diseases

Answer these questions:
What would be the impact on this test's sensitivity if you moved the cutoff for a positive result from A to B? What about specificity? What would happen to sensitivity and specificity if you moved the cutoff from A to C? Where would you put the cutoff for this test? What is the relationship between sensitivity and specificity for any given test? With the expanding knowledge concerning diseases and the further development of technology in diagnostics, screening has become a method in improving the state of public health through its potential for disease prevention. However, people may have misconceptions about this process because they were inadequately informed. This attempts to provide an understanding about the many aspects of screening. It is concerned with how screening is defined, its limitations and advantages, the issues involved and conditions where it is acceptable and not acceptable. 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. According to the reading and establishing a relationship with this case, talking about sensitivity is the percentage of the results that will be positive when HIV is present. On the same way talking about specificity is the percentage of the results that will be negative when HIV is not present.
If we move the cut-off to the left, we can increase the sensitivity, but the specificity will be worse. If we move the cut-off to the right, the specificity will improve, but the sensitivity will be worse. Altering the criterion for a positive test ("abnormality") will always influence both the sensitivity and specificity of the test. As we raise the cut-off level for a positive test, we increase the proportion of normal individuals who correctly test negative, thus increasing specificity. On the other hand, we decrease the proportion of HIV who correctly test positive, thus decreasing sensitivity. I will put the cutoff of this test at the point of not altering or changing any result or influencing the minimum any negative result giving more credibility and reliability to actual circumstances. Although sensitivity and specificity are important considerations in selecting a test, the probabilities they measure are not in themselves what ordinarily concern the physician and the patient after the test result has returned. Both are concerned with the following questions: If the result is positive, what is the probability that disease is present? If the result is negative, what is the probability that the patient is indeed disease-free? These probabilities are known respectively as the predictive value positive and the predictive value negative. They are determined not only by the sensitivity and the specificity of the test, but also by the probability of the disease being present before the test was ordered.
Where would the director of the Blood Bank who is screening donated blood for HIV antibody want to put the cutoff? What would be the potential problem caused by this decision? Where would an investigator enrolling high-risk patients in a clinical trial for an experimental, potentially toxic antiretroviral draw the cutoff? What would be the potential problem caused by this decision? In your answer please detail possible ethical and legal issues of possible concern.
A 1995 study comparing four major brands of HIV tests found that they all had different cut-off points, and as a result, gave different test results for the same sample: “[C]ut-off ratios do not correlate for any of the investigated ELISA pairs,” and one test’s cut-off point had “no predictive value” for any other. (INCQS-DSH, Brazil 1995).
In the UK, if you get through two ELISA tests, you’re positive. In America, you get a third and final test to confirm the first two. The test is called the Western Blot. It uses the same proteins, laid out differently. Same proteins, same nonspecific reactions. But this time it’s read as lines on a page, not a color change. Which lines are HIV positive? That depends on where you are, what lab you’re in and what kit they’re using.

The Mayo Clinic reported that “the Western blot method lacks standardization, is cumbersome, and is subjective in interpretation of banding patterns.” (Mayo Clinic Procedural, 1988). At the same time another study in the Journal of the American Medical Association reported that 19 different labs, testing one blood sample, got 19 different Western Blot results. (JAMA, 260,
1988)

A 1993 review in Bio/Technology reported that the FDA, the CDC/Department of Defense and the Red Cross all interpret WB’s differently, and further noted, “All the other major USA laboratories for HIV testing have their own criteria.” (Bio/Technology, June 1993) In the early
1990s, perhaps in response to growing discontent in the medical community with the lack of precision of the tests, Roche Laboratories introduced a new genetic test, called Viral Load, based on a technology called PCR. An early review of the technology in the 1991 Journal of AIDS reported that “a true positive PCR test cannot be distinguished from a false positive.” (J.AIDS,
1991)

The blood bank medical director is concerned that, because of the low predictive-value positive of the EIA in the blood donor population, the blood bank personnel cannot properly inform those who are EIA positive of their actual antibody status. For this reason, she wishes to evaluate the
Western blot test as a confirmatory test for HIV antibody. The Western blot test identifies antibodies to specific proteins associated with the human immunodeficiency virus. The Western blot is the most widely used secondary test to detect HIV antibody because its specificity exceeds 99.99%; however, it is not used as a primary screening test because it is expensive and technically difficult to perform. Its sensitivity is thought to be lower than that of the EIA.
Because the Western blot test is not yet generally available, the blood bank medical director is wondering whether the initial EIA-positive results can be confirmed by repeating the EIA and by considering persons to have the antibody only if results of both tests are positive. The State
Epidemiologist suggests that they compare the performance of the repeat EIA and the Western blot as confirmatory tests. To do this, they will use the earlier hypothetical sample of 1,000,000 blood donors. They assume that serum specimens that are initially positive by EIA are then split into two portions; a repeat EIA is performed on one portion and a Western blot test on the other portion. Word Cited

INCQS-DSH, Brazil 1995

Bio/Technology, June 1993

JAMA, 260, 1988

http://wwwnc.cdc.gov/eid/article/3/4/97-0401_article.htm

http://www.bmj.com/highwire/filestream/378714/field_highwire_article_pdf/0.pdf