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M09.01.07 Post-Test Probability
Post-test probabilities measure the likelihood of a disease being present or absent after obtaining a diagnostic test result. This analysis involves Positive Predictive Value (PPV) and Negative Predictive Value (NPV), which provide insights into the accuracy of positive and negative test results, respectively.
1. Positive Predictive Value (PPV)
Definition
The Positive Predictive Value (PPV) indicates the probability that a person who receives a positive test result actually has the disease. It is often referred to as the “predictive value of a positive test.”
Formula
Explanation
- PPV specifically measures the distribution of individuals who receive a positive test result.
- A higher PPV indicates a higher likelihood that positive test results are true positives.
| Test Outcome | Disease Present (True Positive) | No Disease (False Positive) |
|---|---|---|
| Positive Test Result | True Positive (TP) | False Positive (FP) |
2. Negative Predictive Value (NPV)
Definition
The Negative Predictive Value (NPV) is the probability that a person who receives a negative test result truly does not have the disease. This is known as the “predictive value of a negative test.”
Formula
Explanation
- NPV measures only the distribution of individuals who receive a negative test result.
- A higher NPV implies a greater likelihood that negative test results are true negatives.
| Test Outcome | No Disease (True Negative) | Disease Present (False Negative) |
|---|---|---|
| Negative Test Result | True Negative (TN) | False Negative (FN) |
3. Accuracy
Definition
Accuracy is the percentage of correct test outcomes (both true positives and true negatives) among all screened patients. It represents the degree to which the measurement correctly reflects the actual presence or absence of disease.
Formula
Mnemonics and Key Concepts
SPIN and SnNOUT
- SPIN: A Specific test that is Positive rules IN the disease.
- SNNOUT: A Sensitive test that is Negative rules OUT the disease.
These concepts represent a trade-off between sensitivity and specificity, often visualized through Receiver Operating Characteristic (ROC) curves.
Points to Remember
- PPV and NPV depend on the prevalence of disease in the population.
- SPIN: Specificity helps confirm a diagnosis with a positive test.
- SNNOUT: Sensitivity is useful to rule out a disease with a negative test.
- Accuracy represents the overall correctness of the test results.
References
- CDC on Predictive Values
- [World Health Organization – Screening for Diseases](https://www.who.int/