Laboratory Testing

Overview Clinicians require accurate laboratory tests to help them diagnose illness and screen for diseases. However, even the best laboratory test is not perfect. It can miss a condition or falsely report a patient as having a condition. Consider the scenario of a rapid strep test (RST) performed in the middle of July on 1000 children. Suppose the probability that a child in this population has a Strep Tonsillitis in this season is 2% (prevalence). If the child has strep tonsillitis, the probability that the RST will catch it is 80% (sensitivity). You are told that the test is 99% specific. What does it mean when a child tests positive? Below is a simplified representation of the information above. In a sample of 1000 children:

As clinicians, it is important to recognize that not all laboratory tests are as perfect as we would like them to be. In order to apply laboratory test results to a specific patient situation, you will need to understand the concepts of reliability, validity, sensitivity, specificity, predictive value, and prevalence of a disease in a given population. This module will help you understand the overarching concepts of laboratory testing and their implications for practice.