Diagnostic Test Selection in an Animal Shelter Setting

Testing is a valuable and often underutilized resource in animal shelters. It can be tricky to decide when to test:

• Not testing can delay answers we need to guide treatment.
• Testing too much can be time intensive and costly.
• Judicious testing allows for efficient diagnosis that guides treatment, protects the population, shortens length of stay, and effectively allocates resources based on risk.

Screening Tests vs. Diagnostics 

Testing in animal shelters can be done on an individual or population level. Individual animal testing is conducted for screening and diagnostic purposes. On a population level, testing helps with disease surveillance, or to diagnose and manage an outbreak.

Screening tests are performed on healthy animals to identify sub-clinical disease when clinical signs are not present. These tests are important to prevent infectious diseases from spreading through the population or to identify diseases with a long pre-clinical phase where early treatment may be beneficial. Screening in shelters is often performed at intake but may be delayed if the condition does not pose an immediate threat to the population.  

A good screening test should provide accurate test results, so animal shelters should invest in tests with high sensitivity and specificity. The prevalence of disease, or the total number of disease cases in a particular community or population, should be taken into consideration when deciding which conditions to screen for and which tests to use. When a screening test is used in a generally healthy population, positive results should be interpreted with caution. Confirmatory testing is often necessary to distinguish between true positive and false positive results obtained on initial screening.

Diagnostic testing is performed on animals displaying clinical signs to guide individual treatment and outcomes. With infectious disease, testing may protect an entire population. Animal shelters should consider whether the test results would change treatment or management plans for the animal or the population. For example, infectious respiratory disease is routinely treated empirically in shelters without performing diagnostic testing on each case. However, testing is recommended: 

• When the severity or duration of clinical signs is different than what is usually seen in that population
• In the face of an outbreak
• When there is concern for highly contagious diseases with high mortality (such as canine distemper)

Sensitivity, Specificity, and Predictive Value

The validity of a test is often reported through its sensitivity and specificity, which reports the percentages of accurate test results in comparison to a “gold standard test.” 

Sensitivity refers to the test’s ability to accurately identify an animal as “diseased.” A test with high sensitivity will have a low number of false negatives. This means most “diseased” animals will test positive, and only a few animals with the disease will be missed due to inaccurate negative results.

Specificity refers to the test’s ability to correctly identify an animal as “disease-free.” A test with high specificity will have a low number of false positives. This means most “disease-free” animals will test negative, and only a few animals without the disease will be incorrectly identified as “diseased” due to inaccurate positive results. 

It is important to understand the predictive value when interpreting results obtained with a particular test in a specific population. Positive and negative predictive values are dependent on sensitivity and specificity of the test being used and on the prevalence of the disease in the population or community.

Predictive values can be thought of as how “trustworthy” the individual test results can be considered. When there is a high positive predictive value, positive test results are generally accurate while negative results should be questioned. If a disease is uncommon in the population, the positive predictive value will be low regardless of the test’s accuracy. When there is a high negative predictive value, negative test results are generally accurate while positive results should be questioned. 

No testing modality is 100% accurate, so a risk-benefit analysis should be completed considering the context and risk of false positive or false negative results. For example, a false positive that results in unnecessary euthanasia or increases length of stay is a highly undesirable result. Screening healthy animals for diseases with low prevalence (for example, FIV and FeLV) may not provide useful test results and, therefore, not be an efficient use of resources. 

Want to learn more about how these principles affect test selection and interpretation? Read more about Basic Epidemiological Principles (PDF).

Sample Collection and Handling

The quality of a test result depends on the quality of the sample collected. Likewise, a sample that is obtained appropriately, but handled poorly, may become useless. Samples must be collected with appropriate sterility, placed in the appropriate containers, and stored appropriately until shipping of the sample. If in doubt, refer to test manufacturer guidelines or contact diagnostic laboratories with questions. 

There are many factors to consider when determining when to test, which test to use, and how to interpret the results. It is important to work with your shelter’s veterinarian to develop protocols around disease screening, especially when testing in the face of infectious disease concerns. These resources are designed to provide answers to   questions that may arise when making these decisions, and if your shelter needs additional support, you can request a shelter consultation with the ASPCA Shelter Medicine Services team.

Resources & Downloads

• Basic Epidemiological Principles (PDF)
• Common Veterinary Diagnostic Tests (PDF)
• Common Veterinary Test Considerations for the Shelter Setting (PDF)
• Infectious Disease Management in Animal Shelters, 2nd ed., Chapter 4

Last Reviewed December 2023