Epidemiology Unit 4, Case Study Example

  1. Table for MRSA Screening
Test/ Actual Positive Negative Total
Positive 30 20 50
Negative 10 1490 1500
Total 40 1510 1550


  1. The prevalence of MRSA based on the screening is 40/ 1550= roughly 2.6 % of the tested population
  2. Sensitivity= 30/40= 75%- the sensitivity of the test is the amount of individuals that tested positive out of the total number of individuals that actually had the disease (determined by the gold standard). This test does not have a high sensitivity rate- that is, out of 100 individuals one will have 25 false positives.  This could lead to extra testing (and related expense).
  3. Specificity= 1490/1510= 98.6% – specificity is the amount of individuals who test negative for a disease out of the total number of individuals that did not actually have the disease (determined by the gold standard). The specificity for the test is high: This means that the test will only have roughly one individual with a false negative (tests negative but is really positive). Because there is a trade-off between sensitivity and specificity, one (could) conclude that the test has adopted a high specificity strategy – this makes sense for MRSA that would likely rather have people deemed as positive (and thus isolated) then have someone tested as negative and then released into the general population.
  4. Predictive value positive

The value for the predictive value positive is:

True Positives/ Number of True positives+ Number of false positives

30/ 30+20= 30/50= 60%


  1. Overall, the metrics of this test are not impressive. The test boasts a low sensitivity and low predictive value positive.  But the final verdict on the test must take consideration of two key factors: 1) this test vis-à-vis other alternatives; 2) the strategy of the test in the population.

Since we do not know the answer to the first issue, we will focus the analysis on the second.  Due to the tradeoff between sensitivity and specificity, if we had to choose, I would want to choose a test with higher specificity than sensitivity.  That is, I would want to prevent against having false negatives (that could lead to a number of infections) rather than false positives that would lead to extra vigilance  against a number of individuals in the hospital.

CDC.  MRSA Infections.

Anderson,K. Epidemiology.  Boston: Case Publishing.