Sensitivity, Specificity, PPV, and NPV
Module 3.2 Synthesis Assignment
Pakistan has seen an increase in cholera cases throughout 2022, however the incidence if new cases has been rapidly increasing following devastating flooding from torrential rains and a heat wave that caused glacial ice to melt. Thirty-three million people have been affected by the recent floods in Pakistan. The disaster has left more than 1,300 people – including more than 400 children – dead, displaced millions and put one-third of the country under water. Cases of acute watery diarrhea (AWD), cholera, and other water-borne diseases are increasing across in the region.
Suppose you are working as part of a team responding to the increase in AWD cases in a small community within the Balochistan province. Your organization is supporting a primary health care center that received a donation of 1,000 rapid diagnostic tests (RDTs) for cholera. The RDTs are performed on stool samples collected from patients with AWD, and the manufacturer states that the sensitivity of the test is up to 93% and the specificity of the test is up to 97%, depending on the setting. Your organization has decided to trial the RDTs and follow-up with laboratory confirmation for all cases of AWD that present to the primary health care center for evaluation and care until all 1,000 RDTs have been used. Both your organization and the manufacturer are interested in knowing the performance estimates for this RDT in this particular setting. You have been tasked with analyzing the data.
All 1,000 RDTs were used on patients with acute watery diarrhea. A stool sample was collected from each patient, and each stool sample was tested with both the RDT and additional laboratory testing for confirmation. Of the 1,000 RDTs conducted, 246 were positive. Of the RDT-positive samples, 200 were confirmed as true cholera cases through additional laboratory testing. The stool samples from the remaining 754 patients evaluated for AWD were negative when tested with the RDT. Of these RDT-negative samples, 30 were confirmed to actually be true cholera through additional laboratory testing.
Use this worksheet to complete your calculations. Then open the canvas assignment and input your answers.
Fill in the 2×2 table below using the information above. All cells in the table should be completed.
True disease status
RDT results Cholera No cholera Totals
Positive
Negative
Totals 1000
After you have completed the 2×2 table, calculate the following performance metrics for the RDT:
Sensitivity
Specificity
Positive predictive value (PPV)
Negative predictive value (PPV)
Using what you know about the impact of prevalence on PPV and NPV, answer the following questions:
What will happen to the PPV if the prevalence of cholera increases in this community?
What will happen to the NPV if the prevalence of cholera increases in this community?
Why will these changes in PPV and NPV occur?
