What are Screening Tests, and Should You Use them?

What is Screening?
Screening is the use of diagnostic tests in apparently healthy individuals with no clinical symptoms.1-2The purpose is to detect asymptomatic disease with the presumption that this allows earlier, more effective intervention and will reduce suffering and delay or prevent death. 

Screening is widespread in human medicine, though it is increasingly a subject of controversy and debate.3Large-scale programs in human medicine to promote testing for specific diseases have been widely used, such as prostate-specific antigen (PSA) testing for prostate cancer and mammography for detection of asymptomatic breast cancer. These are examples of screening programs which are now being questioned and scaled back in light of better evidence concerning their risks and benefits.4-5

However, any diagnostic test employed in an individual with no clinical symptoms of illness is a screening test. In veterinary medicine, there are few large-scale, coordinated screening programs, but routine well-pet exams are a common screening practice. There are also calls for more extensive and intensive screening efforts, such as the recent guideline from the American Animal Hospital Association (AAHA) and the large diagnostic laboratory IDEXX promoting routine lab testing for nearly all pets.6

How Does Screening Benefit Patients?
Most veterinarians understand the potential benefits of screening. Individual patients will benefit if a disease is identified which can be treated more effectively when asymptomatic than at a later stage. Some diseases can be delayed or even cured if treated at an early stage which cannot be so effectively managed if not detected until there are clinical symptoms. Identifying IRIS Stage 2 Chronic Kidney Disease and instituting dietary therapy is an example of effective screening and intervention.7-8This is what most of us assume all screening accomplishes.

True negative results might be considered beneficial as well in that they provide reassurance and, depending on the tests involved, potentially a baseline value that can be used for diagnostic purposes in the future, even though they don’t directly impact morbidity or mortality.

When Is Screening Not Beneficial to Patients?
Screening is of no benefit if it fails to detect disease that is present (false negative results), if it detects disease that is not present (false positive results), if it detects disease that is indolent and would not ever cause clinical symptoms (overdiagnosis), and if it detects disease for which treatment is not effective or no more effective than it would be if the disease had not been identified until the symptomatic stage.

When Does Screening Harm Patients?
Screening is harmful when patients are given an incorrect diagnosis. False positive results create anxiety and discomfort and often incur risk and cost from additional testing or treatment for a disease which does not exist. False negative results provide reassurance when there actually is a disease present, which can delay truly beneficial diagnosis and treatment. 

Even correct diagnoses can cause harm.9-10The distress associated with a diagnosis of cancer in an asymptomatic patient, for example, is not counterbalanced by any benefit if there is no effective treatment. The patient simply lives longer knowing they have an illness they cannot treat. And overdiagnosis, the correct identification of disease that is non-progressive and does not lead to illness, exposes patients and owners to the risks and costs of the initial diagnosis and any further testing and treatment without any potential benefit from intervention. Patients may perceive a benefit from false positive results or from knowing about and treating indolent disease, but objectively this can only cause harm.11-13

How Do We Evaluate the Risks and Benefits of Screening?
It is rarely possible to know if screening has been beneficial or harmful for a particular individual because the outcome in the absence of testing cannot be known. Patients, pet owners, and clinicians nearly always feel as if testing is worthwhile even when not objective benefit can be demonstrated. True negative and false negative results are always reassuring, and if disease develops later there is rarely any recognition that these results may have delayed identification and treatment of it. Even if a false positive test is later shown to be false by further evaluation, many people experience such relief at the eventual result that they are grateful for the screening even though objectively the cost and discomfort associated with it have no possible benefit. Subjective evaluation of screening, then, nearly always supports it, but this is a limited and unreliable measure of the value of such testing.

Statistical evaluation of specific tests can be useful. The well-known parameters of sensitivity and specificity tell us something about the reliability of tests. However, the parameters of positive and negative predictive (PPV and NPV) value are arguably more important clinically. These are a function not only of the tests but of the prevalence of the disease we are testing for. If the majority of the population does not have the disease (as is usually the case with screening of asymptomatic individuals), the PPV will be quite low and most positive results will be false positives. Since most patients diagnosed through screening don’t actually have that disease they are tested for, the overall risks and costs of screening and follow-up testing or treatment may outweigh the benefits even if some individual patients are helped.14

The most accurate way to determine whether more patients benefit or are harmed by screening is through epidemiologic data regarding specific diseases and the outcome of screening and treatment.1-3,10-11,15Data on large populations has shown, for example, that most prostate cancer detected by PSA testing is nonprogressive. While some individuals do benefit from early detection, statistically many more undergo psychological distress (including a rise in heart attacks and suicide) and physical harm (such as incontinence, impotence, and even death) due to testing and treatment. PSA screening is no longer recommended as widely as it once was because such evidence shows the practice does more harm than good.4,9-10Similar evidence has led to reduced use of mammography and many other screening tests in humans.5, 9-10

Unfortunately, there is little data and awareness of this issue in veterinary medicine, and screening is widely viewed as an unqualified good. For example, the recent AAHA/IDEXX effort to encourage more laboratory testing of asymptomatic individuals never mentions overdiagnosis and suggests that even normal results or abnormal findings of no clinical significance should be “celebrated” and treated as useful information.6Screening is frequently promoted as a marker of high-quality, effective medicine.16-19The financial benefits of screening, to veterinary practices and companies that provide testing services, are also often mentioned as a benefit,6without any discussion about the issue of veterinary healthcare costs and the potential impact of this on availability of care.20-21

There is little evidence that screening improves outcomes such as quality of life or mortality in veterinary patients for most conditions. Research shows that testing of asymptomatic individuals often finds abnormalities and that some of these lead to potentially beneficial intervention.6,9However, few studies have looked at costs and risks of screening or gathered objective data on the balance between these and the potential benefits.

Bottom Line
“All screening programmes do harm; some do good as well, and, of these, some do more good than harm at reasonable cost.” (Gray et al, 2008)22The challenge for veterinary medicine is to recognize the potential harms of screening and to actively collect evidence to identify the risks and benefits of specific tests in specific populations. The current approach of assuming the theoretical benefits of screening must apply and that harms are negligible is not consistent with the evidence from human medicine and not a cost-effective, evidence-based approach for improving the welfare of our patients.

 The Power of Stories
Anecdotes are the primary justification for screening in veterinary medicine. The AAHA/IDEXX protocol, for example, includes a section called “The Power of Stories” which provides anecdotes of patients who benefitted from screening tests. Apart from not being an objective measure of the balance between risks and benefits, anecdotes can just as easily be used to challenge screening as to defend it. Here are two brief examples. 

Case Example: 10 Year-old Whippet
The patient presented for an annual examination and was offered a CBC and chemistry panel. Thrombocytopenia was identified, as well as mild neutropenia, elevated amylase, and decrease AST and CPK. Follow-up testing confirmed thrombocytopenia, and the patient was referred for further testing. Tick-borne disease panel, abdominal ultrasound, and thoracic radiographs were unremarkable. A subsequent CBC was unchanged, and pathology review concluded the findings were likely normal for the individual and breed. Despite the lack of any preceding or subsequent clinical illness, the owner experienced great distress at the possibility of serious illness in her pet, and the patient was exposed to the discomfort of numerous diagnostic procedures at a total cost of $924. 

Case Example: 5 Year-old Labrador Retriever
The patient presented with acute-onset lameness in the left hind leg and no other symptoms. Physical examination identified acute cranial cruciate ligament rupture. Pre-anesthetic chemistry panel revealed moderate elevation of ALT. Abdominal ultrasound identified mildly hypoechoic, indistinct nodular foci in the liver. Ultrasound-guided biopsy identified benign nodular hyperplasia. The patient died of hemorrhage following the biopsy. 

References
1. Wilson JMG. Jungner G. (1968) Principles and practice of screening for disease. World Health Organization Public Health Papers #34. Geneva, Switzerland.

2. Speechley M. Kunnilathu A. Aluckal E. et al. Screening in Public Health and Clinical Care: Similarities and Differences in Definitions, Types, and Aims – A Systematic Review. Journal of Clinical and Diagnostic Research. 2017;11(3):LE01-LE04.

3. Bulliard JL. Chiolero A. Screening and overdiagnosis: Public health implications. Public Health reviews. 2015:36(8). 

4. Tabayoyong W, Abouassaly R. Prostate Cancer Screening and the Associated Controversy. Surg Clin North Am. 2015 Oct;95(5):1023-39. 

5. Berry DA. Breast cancer screening: controversy of impact. Breast. 2013 Aug;22 Suppl 2:S73-

6. American Animal Hospital Association. Promoting preventative care protocols: Evidence, enactment, and economics. 2018. Accessed September 21, 2018. Available at: https://www.aaha.org/public_documents/professional/resources/promoting_preventive_care_protocols.pdf

7. International Renal Interest Society. IRIS staging of CKD. 2017. Accessed September 21, 2018. Available at: http://www.iris-kidney.com/pdf/IRIS_2017_Staging_of_CKD_09May18.pdf

8. Ross SJ, Osborne CA, Kirk CA, et al. Clinical evaluation of dietary modification for treatment of spontaneous chronic kidney disease in cats. J Am Vet Med Assoc. 2006 Sep 15;229(6):949-57.

9. McKenzie, BA. Overdiagnosis. J Amer Vet Med Assoc. 2016;249(8):884-889.

10. Welch HG, Schwartz LM, Woloshin S. Overdiagnosed: mak­ing people sick in pursuit of health. Boston: Beacon Press, 2011.

11. Raffle AE. Gray JAM. Screening: Evidence and Practice. Oxford: Oxford University Press, 2007. 

12. Boone D. Mallett S. Zhu S. et al. Patients’ & Healthcare Professionals’ Values Regarding True- & False-Positive Diagnosis when Colorectal Cancer Screening by CT Colonography: Discrete Choice Experiment. PLoS ONE 2017;8(12): e80767. 

13. Brodersen J, Siersma VD. Long-Term Psychosocial Consequences of False-Positive Screening Mammography. Annals of Family Medicine. 2013;11(2):106-115.

14. Maxim LD, Niebo R, Utell MJ. Screening tests: a review with examples. Inhalation Toxicology. 2014;26(13):811-828. 

15. Gates TJ. Screening for cancer: concepts and controversies. Am Fam Physician. 2014 Nov 1;90(9):625-31.

16. Lewis HB. Healthy pets benefit from blood work. Ban­field Data Savant. Accessed September 21, 2018. Available at: www.banfield.com/getmedia/1216c698-7da1-4899-81a3-24ab549b7a8c/2_1-Healthy-Pets-benefit-from-blood-work.

17. Irwin J. Do tests first: pre-surgical blood work may eliminate variety of surprises. DVM 360 Nov 1, 2003.

18. IDEXX Laboratories. Preanesthetic testing: don’t compro­mise—change happens fast. Accessed September 21, 2018. Available at: www.idexx.com.au/smallanimal/inhouse/preanesthetic-testing.html

19. Ward E. Ask the expert: preanesthetic testing in private prac­tice. NAVC Clinician’s Brief. January 2010. Accessed September 21, 2018. Available at: https://www.cliniciansbrief.com/article/preanesthetic-testing-private-practice

20. LaVallee E, Mueller MK, McCobb E. A Systematic Review of the Literature Addressing Veterinary Care for Underserved Communities. J Appl Anim Welf Sci. 2017 Oct-Dec;20(4):381-394.

21. Stull JW. Shelby J. Bonnett B. et al. Broadening access to veterinary care: Barriers and next steps to providing a spectrum of effective healthcare to our patients. J Amer Vet Med Assoc. 2018. In press

22. Gray JAM, Patnick J, Blanks RG. Maximising benefit and minimising harm of screening. British Medical Journal. 2008;336(7642):480-483.

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25 Responses to What are Screening Tests, and Should You Use them?

  1. v.t. says:

    Well, conversely, clients are continuously bombarded with their vet’s “I cannot diagnose effectively without additional tests” (for suspect disease) – so the client is left with two choices: forego additional testing due to costs, and maybe treat “as if” for less dollar amounts (or possibly miss a diagnosis where treatment could have been effective), or, spend an exorbitant amount of money for additional testing that may or may not conclude a diagnosis (then additional costs associated with treatment anyway).

    Likewise, many clinics are now suggesting/including many tests in their annual “wellness” exams, i.e., comprehensive/cbc/chemical profile/thyroid panel/ with or without other add-ons depending on the age of the pet, any suspicion or heading off of age-related disease, etc.

    Just another example, I had a vet who pushed MRI’s (at 600.00, thank you), instead of an ultrasound (at 200.00) where the ultrasound could have been the better diagnostic in many cases. Thankfully, this vet is no longer practicing at the clinic (although I fear for the next clinic she’s employed at).

    Add the costs of medicine, drugs, prescription food, it’s costing more for our pets than it is for ourselves (is $90.00 for a 17lb bag of Rx food really reasonable?!) Oops, getting off topic…

  2. L says:

    “Defensive Medicine”
    Vets are being reported to the board of registration in veterinary medicine when crap happens.
    So I don’t blame them a bit for ordering every test possible to rule out. The consumer can always say no thank you.
    You want them to diagnose without doing a full evaluation, you want to take a chance and then you (the customer) threatens to sue when things don’t go well.

  3. v.t. says:

    L, hope you weren’t commenting on my post? (and you do make a good point!)

    Personally, I’ve screened all my pets and will continue to do so when it is warranted (so many variables here), but maybe I’m fortunate enough to understand the pros/cons moreso than a pet owner who hasn’t worked in vet med and must rely on their vet’s ‘recommendations’.

  4. L says:

    @ v.t.
    No, of course not. I agree with your posts.
    I had a corgi, healthy as could be, other than rabies shots as required by law I avoided annual exams and such.
    Hemangiosarcoma struck at age 9, too late for any treatment options.
    I will always wonder if I had taken her to the vet more often we would have caught it sooner.
    I now err on the side of safety.

    At the end of the day it’s all a crapshoot.

  5. v.t. says:

    L, if it’s any consolation, hemangiosarcoma is nasty, whatever you could have done may not have affected the outcome – I am so very sorry 🙁

  6. skeptvet says:

    Yes, many vets do not understand screening or diagnostic testing very well, and they tend to recommend tests that are not indicated because they fear being criticized or sued for not having run every imaginable test for every possible cause of a problem. The published clinical practice guidelines from AAHA and other sources, which vets are told describe the standard of care they should be expected to provide, almost always recommend extensive screening tests with no evidence for when they are useful and when they are not. The fear of “missing something” is always going to be greater than the fear of running unnecessary tests or of over diagnosis, but we have to at least try to explain to clinicians and pet owners why not doing a test is sometimes the better choice for the patient. An uphill battle, for sure!

  7. skeptvet says:

    I understand this anxiety, but I don’t think it justifies the inappropriate use of diagnostic tests. For one thing, if our testing does more harm than good for our patients, then it is unethical to ignore this just because we are afraid of vindictive litigation. Secondly, the VMB regulatory system is incredibly lax, and I have seen gross malpractice excused over and over again, so I doubt that choosing not to run a diagnostic test when there is good evidence that it is not indicated and when there has been an appropriate discussion about the subject with the owner is ever going to lead a vet to lose a case like this. The evidence in human medicine is absolutely clear that such practices raise costs and harm patients, and defensive medicine is, while psychologically understandable, ultimately not a legitimate justification.

  8. skeptvet says:

    Well, “crapshoot” is going a bit far. There is a lot we don’t know about the value of specific screening tests because we do very little epidemiological research to identify the prevalence of specific diseases in specific populations and to identify over diagnosis when it occurs. The best we can do is be aware of the problem and, as always, try to cautiously interpret how the far greater evidence in human medicine might apply to our patients.

    Anecdotes, as always, cut both ways. I ultrasound my own dog for teaching purposes, found a lesion in the spleen, removed the spleen (it turned out to be a benign, clinically irrelevant lesion) and he died of hemangiosarcoma of the liver several years later. Doing the test and the surgery ultimately did not benefit him and, arguably, cause harm in the form of an unneeded surgery.

    Found exactly the same kind of lesion in a coworker’s dog in thee same way, followed in for two years and there was no change. Ultimately, he died of unrelated issues, In this case, the test caused only unneeded anxiety and followup testing, but at least we avoided the harm of an unnecessary surgery.

    My point is that we don’t know how many cases of over diagnosis there are until we systematically look for them, but our fears will always drive us to do more testing unless (and even if) we have clear evidence that specific tests shouldn’t be done. We have a responsibility to protect our patients by practicing science-based medicine as best we can, and that means not doing tests that don’t help just as much as it means not giving alternatives medicines that don’t work.

  9. v.t. says:

    Skeptvet, thanks for the additional comments.

    I’m so sorry for your loss. I imagine you certainly must have had some suspicion of a potentially sinister problem going on that influenced your decision for surgery for your dog those years earlier. I hope you aren’t putting blame upon yourself for that decision.

  10. Jen Robinson says:

    Excellent article, thanks. I’ve often declined the vet-recommended routine bloods panel for dogs in apparent good health … they never seem to find anything. My physician doesn’t do routine blood work unless there’s a reason to expect high cholesterol or something. Is blood work among the screening procedures you have in mind?

  11. Jazzlet says:

    Thank you skeptvet for a concise, but comprehensive summary of the issues around screening.

    This is an area where there are a lot of questions that many patients/owners do not understand, partly because too many clinicians don’t understand the issues very well. This is especially so if the screening includes scanning like the ultra sound you mention skeptet, or an MRI. When Mr J was given an MRI as a result of a possible lump on one of his kidneys (found by palpation and ultimately benign) they found four or five other oddities that then had to be investigated, most were nothing important, one was a maligant tumour in his appendix that would have been difficult to diagnose acurately any other way, and that would very likely have killed him. In his case that one find probably saved his life, but the appendix cancer occurs at a rate of one in a million, most people would have the worry of the four or five strings of investigation for no gain. As you say we need more data for all of the screening we could do, and to have a better grasp of what that screening actually means in humans and in our pets.

  12. skeptvet says:

    Yes, any diagnostic test run in a patient with no symptoms or indication of a problem is, by definition, a screening test. Bloodwork is probably the most common in vet med. For these tests to be worthwhile, there has to be good evidence of a reason toe run them. In humans, for example, we know people with certain characteristics (age, ethnicity, family history, etc.) are at higher than average risk for certain diseases, so screening for those diseases can make sense. Colon caner is an excellent example of screening proven to benefit more patients than it harms. However, without this data, “fishing expeditions” are rarely productive and can cause harm to patients.

  13. art malernee says:

    Yes, any diagnostic test run in a patient with no symptoms or indication of a problem is, by definition, a screening test. Bloodwork is probably the most common in vet med.>>>

    The annual exam I suspect is the most common screening test in my state.

  14. Jen Robinson says:

    I’m curious. To what extent is excessive screening the vet’s own decision, as opposed to something pushed by practice managers, franchise practices, and economic pressures?
    Relatedly, is there evidence that the annual checkup is worthwhile for young and middle aged, healthy pets (presuming an experienced and observant owner)?

  15. art malernee dvm says:

    I’m curious. To what extent is excessive screening the vet’s own decision>>>
    In the state of Florida the law now requires the pet to have been seen by the veterinarian yearly for a check up to maintain a doctor client patient relationship. Not sure how many states do this now. The change in the Florida regulation occurred on the books about 5 years ago.

  16. L says:

    My vet says “You can’t tell by just looking” and tends to recommend annual labs or bi-annual labs (every 2 years) especially for seniors.
    However if something else is going on either with the dog or the owner ($) at the time that takes priority, he understands.
    I think that’s why it helps to have a good relationship with your vet. You can negotiate if the vet doesn’t have something specific that he needs to rule out.

  17. Jennifer Robinson says:

    Evidence, please. As I understand it, evidence does not favor the annual checkup in human medicine. https://www.nzherald.co.nz/health/news/article.cfm?c_id=204&objectid=10465835
    (NZ reference because I live in NZ and it’s the first relevant page Google pulled up).

  18. L says:

    “As I understand it, evidence does not favor the annual checkup in human medicine”.

    That’s cause insurance companies don’t want to pay for it. I was told prior to my annual this year that I would not need any labs done prior to the visit, that health care was getting away from annual labs to keep cost down.

    So I cancelled my annual, lol. I can take my own BP at CVS, check my weight myself, etc.

    It’s not good for the doctor/patient relationship, in fact I may find a new doctor.

  19. L says:

    I bet the people that can afford boutique medicine are getting all the screening tests they need, lol
    https://www.verywellhealth.com/what-is-boutique-medicine-a-concierge-doctor-practice-2615093

  20. art malernee dvm says:

    for those who follow HH she had a post yesterday called “too many test”
    https://www.skepdoc.info/too-many-tests/
    where she talks about annual exams and other testing.

  21. skeptvet says:

    I think there’s a bit more to it than just insurance industry interests, though that’s always a factor. There is little evidence that annual visits or physical exams improve outcomes in asymptomatic human patients. Veterinary patients may be different in that they cannot self-report symptoms, but there still isn’t any research evidence on the subject.

  22. Elizabeth A. Bergmann says:

    I am wondering what the opinion is for screening electrocardiograms in light of the DCM scare with some cases being possibly related to grain free and legume heavy diets? If a dog has no outward symptoms of DCM and is no longer on a grain free diet, do we consider the extra measure of screening that dog with an echo? A dog may have DCM in the early stages with no outward signs..Some people are also getting Taurine blood levels as well…

  23. skeptvet says:

    An electrocardiogram looks for abnormal heart rhythms, not DCM per se, and would not be an appropriate screening test.

    An echo and taurine levels are recommended for dogs in which there is a concern, but again since the number of cases is VERY small in terms of how many dogs are eating grain-free diets, most of these tests will be negative. I still think they are worth doing in dogs eating the diets so far identified by the FDA or in high-risk breeds (e.g. golden retrievrs), but the testing costs several hundred dollars depending on where you live (both tests together are over $700 in my area), and the echo may not be readily available in places where there are not a lot of cardiologists practicing, so given the uncertainty not everyone chooses to have the tests done.

  24. Sabra Ewing says:

    So are you saying that a dog should not get blood work done every year? I thought this was my first dog, and I only found out that she had kidney failure when she had to get blood work done to get her teeth cleaned. She was acting normally, but then very soon after that, she stopped eating. She didn’t have any type of symptoms before that, and I believe that if I had gotten her blood work done more often, I could’ve known about it. She died when she was four years old. Also, from getting my blood work done, I learned about certain things that I needed to correct. You can learn about things like your cholesterol or vitamin D deficiency. So how are you saying that getting bloodwork would not help? I don’t understand it.

  25. skeptvet says:

    Yes, I am saying that if you run bloodwork yearly on a large number of animals, the results will provide no benefit to most, will help some, and will harm some. In humans, we have a pretty good idea what the relative number of people in each category is for specific blood tests. You don’t get your cholesterol checked annually starting at age 7 because it would be meaningless, but you may start doing it in your forties or fifties depending on your weight, lifestyle, family history of cardiovascular disease, and other factors. The decision about what test to run and when to run it is about understanding the risks and benefits, not just randomly running a bunch of tests at some arbitrary interval “just in case.”

    You might find this book interesting:

    Overdiagnosed

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