In a hypothetical study, 1,000 patients attending a hospital general outpatients department were
tested for diabetes using the two tests below:
Fasting blood sugar [FBS]
Glucose tolerance test [GTT]
There were 100 patients who had a positive GTT and they were classified as true cases of
diabetes. There were also 140 patients with an FBS of at least 6 mmol/l [the cut-off point to
distinguish people with diabetes from those without it]. Among these 140, only 98 had a positive
GTT as well.
a. With this information, please calculate: The sensitivity, specificity and positive and
negative predictive value of the F85 test in this study population.
b. When the cut off point for the PBS was raised to 7 mmol/l, the sensitivity of the test
decreased to 95% and the specificity increased to 98%. Could you please calculate the
predictive value and false negative error rate of the F85 test at this cut-off point?
The FBS and GTT tests were used in a hypothetical community sero-prevalence survey to screen
for diabetes. Among 1,000 surveyed, 40 had a positive GTT for diabetes and were classified as
true cases of diabetes. An FBS cut-off value of 6 mmol/l was used to distinguish between people
with and without the disease. You can assume that at this cut-off point the PBS had a sensitivity
of 98% and an specificity of 95%.
c. What are the positive predictive value and false negative error rate of PBS in this survey?
d. Why is the positive predictive value different from that observed in the hypothetical
hospital-based study?
Assume that if the cut-off point of PBS is increased to 7.5 mmol/l, the sensitivity is 90% and
the specificity is 99% for diagnosis of diabetes.
e. What are the positive predictive value and the negative error rate of PBS if the cut-off
point of 7.5 mmiol/l is used to screen diabetes in this community?
f. If you were asked to fix the cut-off point for PBS for a seroprevalence survey in your
community would you select a 6 mmol/l or 7 mmol/l? Give reasons for your answer,
please.
