1. Figure 1. Table for calculating the accuracy of a diagnostic test.
True Classification
Diagnostic Test
Disease present (+)
Disease absent (-)
Disease present(+)
a = True positive
b = False positive
Disease absent(-)
c = False negative
d = True negative
a + c
b + d

2. Figure 2. Method for calculating sensitivity and specificity of a diagnostic test.
Term
Definition
Calculation
Sensitivity =
% of patients with the disease who test positive =
a/(a + c) x 100
Specificity =
% of patents who do NOT have disease who test negative =
d/(b + d) x 100

3. % of patients with a positive test who have the disease =
Figure 3. Method for calculating positive predictive value (PPV) and negative predictive value (NPV) of a diagnostic test.
Term
Definition
Calculation
PPV =
% of patients with a positive test who have the disease =
a/(a + b) x 100
NPV =
% of patents with a negative test who do NOT have the disease =
d/(c + d) x 100