Diagnostic Testing Considerations for Shelters and Rescues Part One: Fundamentals

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1 Diagnostic Testing Considerations for Shelters and Rescues Part One: Fundamentals Sandra Newbury, DVM National Shelter Medicine Extension Veterinarian Koret Shelter Medicine Program University of California, Davis

National Shelter Medicine Extension Veterinarian

2 What is a diagnostic test?

3 Why test? Protection for people Protection for the population Protection for the Individual animal Prior to investment

4 What is the cost of a wrong result?

5 Not just about disease transmission

6 When to Test: Timing Timing: Before Intake At intake Cool out period Outbreak Response Clinical signs or problems observed Prior to investment Timing Considerations: Minimize waiting time for animals before and after testing Waiting time for results Course of disease Pending investment of resources or emotions Availability of tester.

Considerations: Minimize waiting time for animals before and after testing Waiting

7 Bad news??? No test is perfect

8 To Test or Not to Test? NOT TESTING or accepting false negative Disease spread Failure to treat Heartbreak, angry phone calls, law suits TESTING or accepting false positive Euthanasia or rejection Cost and complications of treatment Cost (time and money) of testing Too much information?

Heartbreak, angry phone calls, law suits TESTING or accepting false

9 Potential for spread Time pressure Limited resources High numbers Political consequences Legal consequences? High stakes

10 Written Protocols: Document Best Practices Written test protocol What are the indications for testing? What is the procedure for testing? Who can perform the test? Document training How are results recorded? Maintain log of all test results

What is the procedure for testing? Who can perform the test?

11 Test Questions Test accuracy? Costs and risks? Which animals will be tested? What actions will be taken based on test results?

12 Test types: what do they detect? Types of tests Wood s s lamp exam Fungal culture Bacterial culture Skin scraping Fecal Float Ear swab and microscopic exam Serum Chemistry Panel / CBC

13 Test types: what do they detect? Antigen test: detects whole or part of infectious organism Antibody test: detects immune response to infectious organism Examples? Parvo, FeLV,, FIV, FIP,, distemper?

14 Accuracy factors for antigen test Positive when: Recent modified live vaccine Cross reaction with similar agent Environmental contamination Negative when: Early or late infection Antigen not present in the sample tested Life stage of agent Antigen/antibody complex Low level infection

Negative when: Early or late infection Antigen not present in the

15 Accuracy factors for antibody test Positive when: Vaccination (killed or MLV) Maternal antibodies Cross reaction Negative when: Immuno incompetence Age Disease Other Recent infection Very late infection

Cross reaction Negative when: Immuno incompetence

16 Other reasons for misleading results Truly infected but may recover Example? Infected with indistinguishable benign agent Example? Truly infected but not necessarily the cause of disease Examples? Inherent limitations to test accuracy And last but not least

17 Incorrect handling of test or sample Most common reason for inaccurate results All instructions MUST be followed exactly Right temperature, time, amount Testing performed only by trained, designated staff Document all results

exactly Right temperature, time, amount Testing

18 And remember Did she say NOT perfect? Labs are not perfect either!

19 Describing inherent test accuracy Sensitivity: : Probability that a diseased animal will test positive Sensitive test few false negatives Specificity: : Probability that a healthy animal will test negative Specific test few false positives

Sensitive test few false negatives Specificity: :

20 Sensitivity and specificity

21 Deciding whether to trust results Positive results are likely to be true when a test is very specific and the disease is common (positive predictive value) Negative results are likely to be true when a test is very sensitive and the disease is rare (negative predictive value)

22 Selecting test population Maximize accuracy Minimize cost Consider: Purpose of test Limitations of test Will it change what you do?

23 Test population: effect on Screening vs. diagnosis predictive value Screening usually lower frequency trust negative more than positive And vice versa

24 Effect of test population FIV test all healthy cats tested: Frequency = 1% Sensitivity = 90% Specificity = 95% Chance that a positive is true = 9/59= 15% Chance that a negative is true =940/941=99.9% FIV pos FIV neg Test Total Test Total

25 Effect of test population FIV test, now only sick intact male cats tested: Frequency = 20% Sensitivity = 90% Specificity = 95% Chance that a positive is true = 180/220 = 82% Chance that a negative is true = 760/800 = 95% FIV pos FIV neg Total Test Test Total

26 Increase specificity to improve PPV FIV ELISA, confirm with Western blot: Sensitivity = 90% Specificity = 99.9% Frequency = 1% PPV = 9/10 = 90% NPV = 989/990 = 99.9% FIV pos FIV neg Test Total Test Total

27 Case definition: effect on predictive value Negative predictive value is improved by calling an animal diseased if either of two tests is positive Positive predictive value improved by calling an animal diseased only if both of two tests are positive

28 Can you be sure? Parvo test now, in sick kittens with vomiting: Frequency = 50% Sensitivity = 50% Specificity = 99.9% PPV = 250/250 = 100% NPV = 500/750 = 66% Parv pos Parv neg Total Test Test Total

29 What else could we do?

30 Interpretation of results Face value versus confirmatory testing Reliability of result in that animal Resource and $ cost of confirmation Consequence of accepting false result Shelter population

31 Documentation Log and medical record Date Animal ID Age, breed, sex, S/N, location Symptoms Test type Result Initials of tester Evaluation of results

32 Thank for your caring! Special thanks to PetSmart Charities for the partnership that makes my position possible. It is people like you who make this possible, over and over again.

33 Resources PetSmart Charities Webinars To sign up to receive notification To view recordings petsmartcharities.webex.com

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