SCREENING - ANSWERS

Screening Problem Bank Answers


Question 1

                                             

Question 2
Primary prevention occurs before the pathological onset of disease, and its aim is to block the start of disease. Secondary prevention takes place from the pathological onset of disease to the occurrence of clini­cal symptoms. Its aim is to delay the onset and duration of symptom­atic disease and improve survival. Tertiary prevention takes place after clinical symptoms develop. Its aim is to slow or block the progression of disease, and so reduce disease sequelae and improve survival.


Question 3
Serious diseases with fairly long and prevalent detectable preclinical phases for which treatment is more effective at earlier stages.


Question 4

	Disease+	Disease-	Total
Test+	840	40	880
Test-	160	460	620
Total	1000	500	1500

a. Sensitivity = a/(a+c) = 840/1000 = 84.0%

Specificity =  d/(b+d) = 460/500 = 92.0%


b. Both metrics assess validity – the performance of a test against an existing gold standard – and while there is no set cut-point for valid vs. invalid, the results are fairly high for both sensitivity and specificity, suggesting that this new test is a reasonably valid test.


Question 5
a. Mislabeling of specimens


b. Low prevalence of disease in the population


c. Poor training of screening technicians


d. Faulty testing equipment


e. Social desirability bias


Question 6
A.
Sensitivity = 414/414 = 100%.

100% of those with glaucoma tested positive using the new intraocular pressure test.


Specificity = 580/799 = 73%.

73% of those without glaucoma tested negative using the new intraocular pressure test.

B.
Sensitivity = 381/414 = 92%.

The probability of testing positive for intraocular pressure among persons who truly have glaucoma was 92%.


Specificity = 796/799 = 99.6%.

The probability of testing negative for intraocular pressure among persons who truly do not have glaucoma was 99.6%.



Positive predictive value [PPV]: 381/384 = 99.2%.

Among those who tested positive for intraocular pressure, 99.2% truly had glaucoma.     


Negative predictive value [NPV]: 799/829 = 96.0%.

Among those who tested negative for intraocular pressure, 96% truly did not have glaucoma.     


Question 7
A. Sensitivity is a measure of a screening test's validity. It is the prob­ability that a screening test classifies as positive those individuals who have preclinical disease.

B. Specificity is also a measure of a screening test's validity. The complement of sensitivity, it is the probability that a screening test classifies as negative those individuals who do not have preclinical disease.

C. Predictive value of a positive test is a way to measure a screening program's feasibility. It is the proportion of individuals with a posi­tive test who have preclinical disease.

D. Lead time is the amount of time that the diagnosis of disease is advanced by screening.


Question 8
The three types of bias are lead-time bias, length-bias sampling, and volunteer bias. Lead-time bias means that survival among screened individuals may appear longer than that for nonscreened individuals just because they were diagnosed earlier. Length-bias sampling means that survival among screened individuals may be longer than that for nonscreened individuals because screening tends to identify less aggressive forms of the disease. Volunteer bias is a form of confounding that occurs only in observational studies of screening programs. It means that people who get screened tend to have different characteristics than those who do not get screened. These characteris­tics may be related to survival.