The diagnostic process for infectious disease diagnosis in animals

Veterinary diagnostic laboratories provide a vast catalog of tests for infectious agents and often have multiple tests and testing modalities for individual pathogens. Understanding which test to select, which sample type is best, and which animal to sample in a population are critical factors for drawing appropriate conclusions and diagnosing a disease. Each diagnostic test has its inherent diagnostic sensitivity and specificity, and, in many situations, multiple tests may be required for proper interpretation. When multiple tests are needed in a large population (eg, animal shelters, farmed animals, and breeding colonies), the cost of diagnostic testing becomes an increasing concern, and knowing when and how to use aggregate or composite samples is critical. A clinical examination is the first and arguably most important diagnostic procedure providing the framework for laboratory test selection. This yields not only a differential diagnosis but an expected stage of disease, which is necessary for determining the most appropriate laboratory tests and how they should be interpreted. This review will discuss the diagnostic process, strengths and limitations of commonly requested tests at a diagnostic laboratory (eg, histopathology, microbial culture, PCR, and ELISA), common sources of error in interpretation, the impact of pooling samples on different tests, and ways to help strengthen conclusions made from diagnostic test data.