BIO SCI 45 Lecture Notes - Lecture 16: Positive And Negative Predictive Values, Window Period

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28 May 2016

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In order to measure the validity, we have to administer both a gold standard and a screening test. With hiv, the gold standard is not invasive. Sensitivity: proportion of those with disease who test positive in the screened group = a/a(a+c) or tp/ (tp+fn) Specificity: ability to identify true negatives; confirm absence of disease. In order to evaluate the validity of a clinical test, a gold-standard for diagnosis is required. High sensitivity may come at the expense of many false positives. High specificity may come at the expense of many false negatives. The first elisa tests detected 96 98 percent of hiv-infected blood samples. Step #1: conduct a less expensive/less invasive test. Step #2:conduct a 2nd test among those who tested + Re-screen positives with a more specific test (western blot) Effect of re-screening those who initially tested positive: lose overall sensitivity. In a clinical setting, we are more interested in predictive values.

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3. The standard blood test screen for the Human Immunodeficiency Virus (HIV) detects antibodies to the virus using a laboratory test known as ELISA (enzyme-linked immunosorbent assay). If the ELISA test is run properly under recommended conditions, it has a sensitivity of 99% and a specificity of 99%.

a) (4 pts) Assume the HIV-ELISA test is administered to a low-risk community of 100,000 people. By low-risk we mean a population which has an HIV prevalence of 1%. Construct the appropriate 2 X 2 table, determine the number of false positives and false negatives, and calculate the positive predictive value of the ELISA test in this population.

b) (4 pts) Assume the same ELISA test with the same sensitivity and specificity is used to screen a high-risk population (a population of 100,000 people with an HIV prevalence of 12%), Construct the appropriate 2 X 2 table, determine the number of false positives and false negatives, and calculate the positive predictive value of the ELISA test.

)A screening test is available to identify individuals with a genetic mutation that puts them at greater risk for stroke. The test is extremely expensive and is not covered by most insurance plans, so those choosing to have the test usually have to pay for it out-of-pocket. However, it has been determined that the screening test is very valuable because the annual incidence of stroke among those who have been screened is nearly half that of the general population. 7) One major benefit of mammography screening for breast cancer is that the disease is often diagnosed in the presymptomatic phase of the disease, before it is detectable by any other means, and breast cancers diagnosed in such early stages respond better to treatment. Another benefit of screening is that women diagnosed with breast cancer by way of mammography have better survival rates than women diagnosed by other means. 8) The prognosis for neuroblastoma, a tumor that occurs in young children, varies greatly by patient. Some tumors are slow-growing and may even heal spontaneously. Others tumors are fast-growing and aggressive. A urine screening test for neuroblastoma is currently available that detects metabolites produced by the tumor. It has been suggested that such screening is highly valuable PH20001 Essentials of Epidemiology 2 because the case-fatality rate of neuroblastoma among children diagnosed with the urine test is much lower than it is for those who were not screened. 9) Suppose that a new screening test has recently been developed to aid in the early identification of individuals with rheumatoid arthritis. However, the value of the test has not yet been established. In an effort to test the effectiveness of the new test in predicting a future diagnosis of rheumatoid arthritis, investigators enroll a group of volunteer participants, record their test result, and follow them forward for the development of the condition. It is subsequently determined that the new screening test is highly effective in predicting a definitive diagnosis of rheumatoid arthritis because the incidence of the condition among those who screened positive was much higher than that of the general population. 10) There are at least a dozen types of human papillomaviruses (HPV) that are considered high-risk and cause approximately 5% of all cancers world-wide. However, high-risk HPV infections are common and most occur without any symptoms and resolve on their own within 1-2 years. In 2003, cervical HPV screening tests became available and are now approved for use in combination with a Pap test among women over the age of 30. Such tests can detect infection of several high-risk HPV types before cell changes become evident. Some argue that HPV screening tests should be approved for universal use among women of all ages because the incidence of high-risk HPV infections has increased dramatically over the past decade. Identify the bias being described in the scenario

taupelocust180

1. The risk of acquiring disease is measured by the

a. Incidence rate

b. Incidence rate times the average duration of the disease

c. Incidence rate divided by the prevalence rate

d. Prevalence rate

e. Prevalence rate times the average duration of the disease

2. The strength of an association between a factor and a disease is best measure by .

a. Incubation rate

b. Incidence of the disease in the total population

c. Prevalence of the factor

d. Attributable risk

e. Relative risk

3. Case fatality rate for a given disease refers to

a. The crude mortality rate per 100,000 population

b. Cause-specific mortality rate due to the disease

c. A fatal outcome of any disease

d. The percentage of deaths among cases of the disease

e. The proportion of deaths due to the disease among all deaths from all causes

4. An investigator is interested in etiology of neonatal jaundice. To study this condition, he selected 100 children who were diagnosed with this condition and 100 children born in the same period and in the same hospital who did not have a diagnosis of neonatal jaundice. He then reviewed the obstetrical and delivery records of their mothers to determine various prenatal and perinatal exposures. This is an example of a(n):

a. Cross-sectional study

b. Case-control study

c. Cohort study

d. Clinical trial

e. Experiment

5. Which of the following statements describes the major advantages of a randomized clinical trial?

a. It avoids observer bias

b. It lends itself to ethical justification

c. It yields results replicable in other patients

d. It rules out self-selection of participants to the different treatment groups

d. It enrolls representative patients

6. A survey conducted in England revealed that of 224 families in which there had been a known case of poliomyelitis, 56 maintained parakeets as a family pet. In another British survey, 30 out of 99 poliomyelitis patients questioned kept parakeets. The inference that there is some relationship between the presence of a parakeet in a household and the occurrence of poliomyelitis among household members is

a. Correct b.

Incorrect because of failure to distinguish between incidence and prevalence

c. Incorrect because a proportionate ratio is used when a rate is required to support the inference

d. Incorrect because a failure to recognize a possible cohort phenomenon

e. Incorrect because there is no control or comparison group.

7. An investigator determines the correlation coefficient between triglyceride levels and degree of atherosclerosis in sampled blood vessels to be +1.67. On the basis of this you would conclude that:

a. Triglyceride level is a good predictor of atherosclerosis

b. Triglyceride level is not a good predictor of atherosclerosis

c. High triglyceride levels cause atherosclerosis

d. Atherosclerosis cause high triglyceride levels

e. The investigator has incorrectly determined the correlation coefficient

8. A screening test of known sensitivity and specificity is applied to two populations. The prevalence of the disease being screened for is 10% in population A and is 1% in population B. Which of the following is true?

a. The percent of all negative tests that have false-negative results is lower in population A than in population B.

b. Specificity is lower in population A than in population B.

c. Reliability is higher in population A than in population B.

d. The percent of all positive tests that have false-positive results is lower in population A than in population B.

e. Specificity is higher in population A than in population B.

9. Serum cholesterol levels are obtained for four healthy men. The probability that all will fall below the 10th percentile of the distribution of cholesterol for healthy males is:

a. 0.4

b. 1 â (0.1)4

c. (0.1)4

d. (0.9)4

e. Cannot be determined from these data

10. In a diabetes screening program, the screening level for a positive blood sugar level in test 1 is set at 160 mg/dl, and in test 2 at 130 mg/dl. The sensitivity is:

a. Greater in test 1

b. Greater in test 2

c. Equal in test 1 and 2

d. Dependent on the size of the population being evaluated

BIO SCI 45 Lecture Notes - Lecture 16: Positive And Negative Predictive Values, Window Period

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