Learning Objectives
- Define the parameters of clinical screening efficacy.
- Differentiate between the four test result qualifiers (TP, FP, TN, FN).
- Construct a 2×2 contingency table using raw clinical data.
- Apply the “Gold Standard” concept to validate new screening tools.
I. Principles of Screening
Screening tests are used to detect the potential presence of disease in asymptomatic individuals. Their efficacy is determined by comparing results against a Gold Standard (the most accurate test available).
Example: In HIV diagnostics, the ELISA acts as the screen, while the Western Blot is the gold-standard verification.
II. The Qualifier Matrix
The “True/False” labels describe the correlation between the test result and the actual disease state.
| Qualifier | Meaning | Clinical Reality |
|---|---|---|
| True-Positive (TP) | Test (+) is True | Patient is Sick |
| False-Positive (FP) | Test (+) is False | Patient is Healthy |
| True-Negative (TN) | Test (-) is True | Patient is Healthy |
| False-Negative (FN) | Test (-) is False | Patient is Sick |
III. The 2 × 2 Table (Measures of Performance)
This table is the primary tool for calculating sensitivity, specificity, and predictive values. Always place Disease State on the columns and Test Results on the rows.
| Screening Result | Disease (Gold Standard) | Row Totals | |
|---|---|---|---|
| Present | Absent | ||
| Positive (+) | TP (60) |
FP (70) |
130 (TP + FP) |
| Negative (-) | FN (40) |
TN (30) |
70 (TN + FN) |
| Column Totals | 100 (TP + FN) | 100 (TN + FP) | 200 (Grand Total) |
Step 3 Tip: When given a clinical vignette, quickly sketch this table. Note that the sum of the first column (TP + FN) represents all sick people, while the sum of the second column (TN + FP) represents all healthy people.