Results gathered during the development and testing of an assay must be analyzed to determine the performance of the assay. In addition, the performance must be gauged by clinical parameters. Key elements include:
Assay parameters
- Sensitivity - The percentage of patients with the disease and a positive test.
- Specificity - The percentage of patients without the disease and with a negative test.
Clinical parameters
- Positive predictive value (PPV)- the probability of disease if the test is positive
- Negative Predictive value (NPV)- the probability of no disease if the test is negative
- Prevalence - How much of a given disease (or test component) is in the population. Note: The PPV and NPV are affected by the sensitivity and specificity of a given assay and its prevalence. For example, one would follow up with a patient in a population with extremely low prevalence with a positive test.
The table below details a standard 2 X 2 comparison, where assay results are compared to a standard or a specific disease state.
| STANDARD (DISEASE STATE) | |
Positive | Negative |
ASSAY RESULTS | Positive | a | b | a+b |
Negative | c | d | c+d |
| a+c | b+d | n |
a = true positive, the patient has the disease, and the test is positive
b = false positive, the patient does not have the disease, but the test is positive
c = false negative, the patient has the disease, but the test is negative
d = true negative, the patient does not have the disease, and the test is negative
n = total tested
Calculations to assess assay performance:
- Sensitivity % = a/(a+c) x 100
- False negativity % = c/(a+c) x100
- Specificity % = d/(d+b) x 100
- False positivity % = b/(b+d) x 100
- Predictive value positive %: a/(a+b) x 100
- Predictive value negative%: d/ (d+c) x 100
