M09.01.05 Screening test

Screening tests are essential tools in healthcare to detect the presence or absence of a disease in asymptomatic individuals. For instance, the ELISA test is commonly used to screen for HIV infection, with test results indicating either a positive or negative status. The accuracy of these tests is often validated by comparing test outcomes against a gold-standard test, such as the Western blot for HIV.

Key Definitions in Screening Tests

Screening tests are evaluated based on true-positive, false-positive, true-negative, and false-negative outcomes. These terms help physicians understand the relationship between test results and the actual disease status.

Outcomes of Screening Tests

1. True Positive (TP):
   • The test result is positive, and the individual truly has the disease.
   • Interpretation: The positive result accurately indicates disease presence.
2. False Positive (FP):
   • The test result is positive, but the individual is healthy.
   • Interpretation: The positive result incorrectly indicates disease presence.
3. True Negative (TN):
   • The test result is negative, and the individual is truly disease-free.
   • Interpretation: The negative result accurately indicates disease absence.
4. False Negative (FN):
   • The test result is negative, but the individual has the disease.
   • Interpretation: The negative result incorrectly indicates disease absence.

2 × 2 Screening Test Outcome Table

The efficacy of a screening test is often analyzed using a 2 × 2 table that compares the disease status and test results. The table below organizes the outcomes for each combination of test results and disease presence or absence.

• True Positive (TP): 60 cases
• False Positive (FP): 70 cases
• True Negative (TN): 30 cases
• False Negative (FN): 40 cases

Points to Remember

• Screening tests are not always definitive; they require follow-up confirmatory testing.
• True positives and true negatives are correct predictions, while false positives and false negatives indicate test errors.
• The gold standard test, such as Western blot for HIV, confirms the results of the screening test.

References

1. Jones, T. & Bartlett, R. (2021). Principles of Disease Screening and Testing. Academic Press.
2. Smith, J. & Doe, M. (2020). “Understanding Test Accuracy in HIV Screening,” Journal of Infectious Diseases, 15(3), 456-478.