However, a gold standard for the proof of infection is missing. Sepsis may be seen as systemic inflammatory response due to infection. Worldwide, sepsis and its sequelae still remain a frequent cause of acute illness and death in patients with community and nosocomial acquired infections. In order to show how to apply these concepts in practice we explain how to use the freely available software R. Finally we discuss sample size estimation for diagnostic studies. Procalcitonin is actually a continuous biomarker, hence we introduce the use of receiver operator curves (ROC) and the area under the curve (AUC). This can be improved using natural frequencies which are applied to our example. Predictive values are sometimes difficult to communicate. Then, we introduce predictive values using Bayes’ theorem. For these measures the construction of confidence intervals is demonstrated. Next likelihood ratios are introduced which combine the information given by sensitivity and specificity. As an example we use Procalcitonin with a cut off value ≥ 0.5 g/L as a test and Sepsis-2 criteria as a reference standard for the diagnosis of sepsis. The validity of a diagnostic test can be assessed using sensitivity and specificity which are defined for a binary diagnostic test with known reference or gold standard. This tutorial gives an introduction into statistical methods for diagnostic medicine.
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