ISSN (on-line): 1806-3756 | ISSN (printed): 1806-3713
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CONTINUING EDUCATION : SCIENTIFIC METHODOLOGY
Understanding diagnostic tests. Part 2.
Entendendo os testes diagnósticos: parte 2
Cecilia Maria Patino1; 2; Juliana Carvalho Ferreira1; 3
1. Methods in Epidemiologic, Clinical, and Operations Research-MECOR-program, American Thoracic Society/Asociación Latinoamericana del Tórax, Montevideo, Uruguay.
2. Department of Preventive Medicine, Keck School of Medicine, University of Southern California, Los Angeles (CA) USA.
3. Divisão de Pneumologia, Instituto do Coração, Hospital das Clínicas, Faculdade de Medicina, Universidade de São Paulo, São Paulo (SP) Brasil.
PRACTICAL SCENARIO
Investigators studied the diagnostic accuracy of serum procalcitonin levels in diagnosing parapneumonic pleural effusions (PPE) and differentiating it from other causes of pleural effusions. They found that procalcitonin had a positive predictive value (PPV) of 66% and a negative predictive value (NPV) of 91%.(1)
PPV AND NPV OF DIAGNOSTIC TESTS
In the previous article(2) we discussed two common features of diagnostics tests, sensitivity and specificity, which are important characteristics that describe the accuracy of a test. In this article, we focus on important features of a diagnostic test that help us understand how well a new test diagnoses a disease based on the results of the gold standard: PPV and NPV.
The PPV of a diagnostic test is the proportion of individuals who test positive to the new test and have the disease according to the gold standard (the proportion of true positives). When a diagnostic test has a high PPV, there is a high probability that a patient has the disease being investigated when the patient has a positive test.
The NPV of a diagnostic test is the proportion of individuals who test negative to the new test and do not have the disease according to the gold standard (the proportion of true negatives). When a test has a high NPV, there is a high probability that a patient does not have the disease being investigated when the patient has a negative test. In our example, the PPV was 66% (39/59), and the NPV was 91% (81/89), according to the results of the new test among 148 individuals (Table 1).
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