In recent years, the calculation of predictive values has come into importance.
Predictive values relate to the actual disease or illness prevalence (the proportion of a particular population affected by the medical condition over a specified period of time). As time passes, the lab test in question becomes more widely used, improving the data of epidemiology (determinants/distribution of disease in a defined population). When the disease prevalence is high, the lab test is better at 'ruling in' than 'ruling out' the disease. (PPV increases as prevalence increases.) Therefore, providers, researchers, and laboratorians must understand the prevalence of the actual disease or illness in their population when making medical decisions or choosing a test.
Improved test accuracy (as reflected by sensitivity and specificity) also influences - and enhances - the predictive value of a test.2
Shreffler and Huecker describe the predictive value and calculations in the following sentences.3
- Positive predictive value (PPV): The higher the disease prevalence, the higher the predictive value of a positive test.
To determine PPV percent, use the following equation:
True Positives (TP)
Divided by True Positives (TP) plus False Positives (FP)
Times 100
or
[TP ÷ (TP + FP)] x 100
- Negative predictive value (NPV): The higher the disease prevalence, the lower the predictive value of a negative test. (Note: This effect may be small when test sensitivity and specificity are high.)
To determine NPV percent, use the following equation:
True Negatives (TN)
Divided by True Negatives (TN) plus False Negatives (FN)
Times 100
or
[TN ÷ (TN + FN)] x 100
2. McPherson RA, Pincus MR, eds. Henry's Clinical Diagnosis and Management by Laboratory Methods. 24th ed. Elsevier; 2021. (Chapter 8.)
3. Shreffler J, Huecker MR.
Diagnostic Testing Accuracy: Sensitivity, Specificity, Predictive Values and Likelihood Ratios. [Updated 2023 Mar 6]. Online; In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan. Available from:
https://www.ncbi.nlm.nih.gov/books/NBK557491/
