4 Diagnostic Testing

       Brian A. DiGangi

       American Society for the Prevention of Cruelty to Animals (ASPCA), New York, NY, USA

       For most diagnoses all that is needed is an ounce of knowledge, an ounce of intelligence, and a pound of thoroughness.

      – Anonymous

4.1 Introduction

      A diagnostic test can be defined as any tool used to obtain information that enables the practitioner to identify a disease or condition. In veterinary medicine, diagnostic tests typically include physical examination findings, information gleaned from interviewing an owner or caretaker, history of the patient and its relatives or cohorts (i.e. anamnesis), and paraclinical findings reported by laboratory tests and imaging studies (Blood and Studdert 1999). Determining the cause of disease enables the veterinarian to prescribe appropriate treatment and render a prognosis. In the face of an infectious disease outbreak, obtaining a diagnosis can guide management decisions to mitigate the impact of the outbreak. Of particular importance in the shelter setting, obtaining a diagnosis also ensures efficient resource allocation and maximizes the quality of life for shelter animals.

      In addition to identifying the cause of an acute or chronic disease, diagnostic testing may also be utilized to identify both animals with subclinical infection and those that are free from infection (Evermann et al. 2012). Such distinctions allow the shelter veterinarian to determine the risk of co‐housing individual animals and to develop rational biosecurity protocols to prevent the introduction and spread of disease within the shelter environment.

      Diagnostic tests for infectious diseases can be broadly categorized as direct or indirect. Direct testing methodologies rely on detection of the pathogen itself (e.g. culture, polymerase chain reaction [PCR]), while indirect testing methodologies focus on detecting the body's response to the pathogen or disease process (e.g. antibody testing, biochemical profile). It is important for the practitioner to understand the testing modality that is being used, as well as its limitations. For example, direct testing methods do not necessarily indicate whether the pathogen identified is dead or alive, nor can they always quantify the severity of the disease. Similarly, indirect testing methods often cannot distinguish between current or previous exposure to a pathogen or active disease.

In the face of resource limitations common in many shelter organizations (i.e. time, money, training, expertise), the utility of a comprehensive diagnostic testing strategy is often overlooked or considered unattainable; however, such limitations are not a reason to ignore best practices. For many practitioners, the joys and challenges of shelter practice lie within creatively meeting the spirit of those practices in the face of limited resources. Thoughtfully employed diagnostics can lead to cost‐effective protection of animal health and welfare—a goal that is shared by everyone.

4.2 Obtaining Diagnostic Samples

Every organization with a medical program should strive to maintain some minimum diagnostic testing supplies and equipment for an in‐house laboratory. See Box 4.1. Such items are commonly available through veterinary and medical supply distributors at minimal cost (<$500 for disposable supplies listed in Box 4.1). Inexpensive, gently used diagnostic equipment can often be found online or through local human hospitals. Maintaining an in‐house laboratory will allow the practitioner to establish a presumptive or definitive diagnosis in the majority of infectious diseases common in animal shelters. An in‐house laboratory also supports the ability to perform all the diagnostic procedures described in the sections on core diagnostic tools and primary diagnostic testing, and many of the procedures in the section on secondary diagnostic testing. Additional supplies and equipment can be obtained when expanding the scope of an in‐house laboratory or as needed for the routine collection of samples for submission to diagnostic laboratories. In many cases, diagnostic laboratories will provide sample or test‐specific collection equipment at no charge. Care should be taken to ensure that the type and number of samples, the manner of collection (i.e. sterility), along with the collection container (including preservatives or additives) are appropriate for both the type of diagnostic test to be conducted and the interpretation of its results.

Box 4.1 Minimum Recommended Diagnostic Testing Supplies and Equipment

      Supplies

      Syringes (1 cc, 3 cc)

      Needles (21–25 gauge (g) x 1″)

      Blood collection tubes (Sterile “red top,” EDTA “purple top”)

      Microhematocrit capillary tubes and tube sealant pad

      Microscope slides

      Romanowsky‐type stain (e.g. Diff‐Quik, Siemens Healthcare Diagnostics Ltd., Deerfield,