Veterinary point-of-care tests: Evidence-based insights

By Erica Tramuta-Drobnis, VMD, MPH, CPH

Point-of-care tests (POCTs) pertain to laboratory testing performed outside the boundaries of a send-out reference laboratory, usually in close proximity to patients. These types of diagnostics have revolutionized veterinary medicine. Rapid results permit faster patient care and provide swift answers for clients worried about their furry friends. However, not all POCTs are created equal. Some diagnostic tests consistently produce reliable results, while others are more suspect.

POCTs: Are they right for you?

As there is no governmental oversight in clinical veterinary laboratory medicine, veterinarians are ultimately responsible for quality control (QC) and quality assurance (QA), both with our large diagnostic laboratories and POCTs. While these tests can be practice-changing, not all are inherently valuable. Several of these tests have proven to be of much less utility over time. Others have gray zones that do not allow for accurate interpretation and still require sending blood out. Some may have no value at all.

Key factors in determining whether a practice should utilize a specific POCT and or judge one's merits include:¹

• Adequate operator training to ensure compliance and proper performance
• Rigorous standard operating procedures (SOPs)
• Proper QC guidelines
• Routine maintenance and assessment of instrument performance
• Proper reporting of results
• Proper controls to assess the validity of the results
• Periodic comparison of POCTs with outside labs

When evaluating any test, the following factors must
be considered:

• The accuracy of a test (how closely the result equals the actual value)
• The analytic range (the normal reference range without dilution or other changes to the sample)
• The control (material used to ensure QC)
• Proper calibration (used to adjust/refine/define a procedure's measurement)
• A well-defined and established limit of detection (lowest level of analyte detectable within a sample)
• Sufficient method validation (studies supporting benefit and effectiveness with supporting materials for detection limits, reference method, range, interferents, and precision)
• Confirmed precision (repeatability/reproducibility of the test–if the same sample is analyzed numerous times, we get the same result)
• Repeatability
• Reproducibility

When POCTs may be valuable

POCTs provide value in numerous clinical settings and circumstances, including:

• For immediate (or close-to) results (e.g. in emergency and specialty centers)
• Pre-surgical bloodwork on the day of a surgical procedure
• Where the ability to send to a lab may be limited due to distance, location, time, or cost.
• Where inclement weather conditions prohibit the timely shipment of samples
• Where immediate clinical and/or therapeutic decisions are needed 

  

Which POCTs can we trust?

Methods of analysis vary from test to test and assay to assay. Methods used include antibody or antigen-based testing. Others utilize nucleic acid amplification (NAAT); other modalities are being developed regularly. However, validation studies with sufficient sample sizes, sufficient quality control studies, and numerous other limitations impact our ability to ensure the true worth of many of our tests. Additionally, factors, such as interference with and lack of quantitative measures (SNAP cPL) further challenge the value of some POCT.^2,3

In-house diagnostic equipment for routine CBCs and chemistries is likely reliable. Because CBC and chemistry machines are frequently utilized and most use validated systems and QC measures, they are likely to be diagnostically helpful and relatively accurate, assuming appropriate care and maintenance are maintained.

However, what about other bedside tests? Consider the following and the evidence for them.

1) Bedside pro-BNP. Synthesized by atrial myocytes, B-type or brain natriuretic peptide (BNP) is a prohormone (proBNP). Physiological stretching of the atria leads to the splitting of the proBNP hormone. This results in two smaller peptides: ⁴

• The inactive N-terminal peptide (NT-proBNP)
• A physiologically active C-terminal peptide (C-BNP)

With the progression of cardiac disease, the hormone is increasingly produced. C-BNP functions to mitigate the stretch that prompted its release. The hormone interacts with receptors in the blood vessels to elicit and facilitate permeability, encouraging diuresis and natriuresis via renal excretion.⁴ The production quantity is thought to correlate with the increasing severity of clinical disease.

POCTs for pets that evaluate proBNP help assess the levels of NT-proBNP in the pet's system. The presumption is this measure serves as a surrogate indicator for augmentations in the dimensions of the atria and ventricles and demonstrates wall tension levels.⁴ According to cardiologists, proBNP testing in cats and dogs may assist in identifying those with a murmur or arrhythmia with occult underlying heart disease.⁵

Radiographs and other diagnostics may help move our decision tree towards or away from primary respiratory vs. primary cardiac conditions in patients with acute respiratory signs. Positive POCT results may incentivize owners to seek advanced diagnostics such as ECG and echocardiography in the face of clinical signs. However, the test is not recommended for routine wellness monitoring, though it is now included in many diagnostic laboratory panels. This may lead to confounding information, unnecessarily increased patient testing, and unwarranted client expenses.^6,7

A study with 217 overtly healthy cats concluded while positive results suggest further testing is warranted, negatives can give false security and do not reliably exclude heart disease.^6,7

In a study with 115 dogs demonstrating signs of respiratory distress, serum NT-proBNP levels evaluated demonstrated a sensitivity of 85.5 percent with a specificity of 81.3 percent in showing those with congestive heart failure (CHF) over those with underlying respiratory disease as the cause of signs. While this study did not use the bedside test, it suggested in-house testing may provide value.⁸ Additional studies also suggest the test has utility when confirming or refuting the presence of CHF.^{9, 10}

Confounders in the use of pro-BNP testing in small animals (bedside or otherwise) may include:⁵

• Underlying disease, including hypertension (pulmonary/systemic), chronic kidney disease, and infectious conditions
• Any medication given that affects overall volume status, e.g. a diuretic
• Normal weekly variation
• Proper handling and processing of samples. The stability of the canine proBNP is both temperature and time-dependent

The proBNP test may provide valuable information at the POC in conjunction with radiographs, clinical pictures, + /—echocardiogram, and ECG. However, owners should be informed the test cannot be relied on as a stand-alone test and negative results do not fully rule out heart disease.⁷

2) In-house cortisol. Project ALIVE started in 2016 with funds from the European Society of Veterinary Endocrinology (ESVE), to help explain various "hot topics" in the field of endocrinology pertaining to animals. The goal was to help establish consensus statements and make evidence-based recommendations.¹¹ The collective recommendation from this group is:

"ALIVE recommends employed cortisol assays should be validated and subjected to quality control; usually this means tests should be run by reference laboratories and not performed in-house to be reliably accurate."¹²

Both false positive and false negative results are possible from unvalidated POCTs for cortisol. This can lead to misdiagnoses, including:

• Misdiagnose a dog with hypoadrenocorticism or Cushing's disease
• Incorrectly presume a patient being treated for Cushing's is well-controlled

While some veterinarians may use cortisol POCTs to make clinical care recommendations, a quick search of the literature proves challenging in finding sufficient evidence to support or refute the use of this test. Further validation studies and criteria for use are warranted.

3) SNAP lepto test. Leptospirosis, an infection most commonly recognized in the urine of wildlife, is increasingly demonstrating itself as a problem in small animals, regardless of size, lifestyle, or breed. Because of its zoonotic nature, rapid diagnostics and immediate institution of therapy are warranted.

However, while the SNAP lepto test is theoretically valid, it clearly states in the product guide in numerous languages the product cannot differentiate between wild type and vaccination strain. Thus, this diagnostic test proves to have little value in regions that regularly vaccinate against lepto.³ A positive test in a dog that has never been vaccinated against leptospirosis in its lifetime (which is much more challenging to determine than we would hope) is valuable. If an animal's vaccination status is unknown, a Leptospira PCR test should be considered.

4) Witness Leptospirosis test. The WITNESS® Lepto test by Zoetis uses a rapid IgM assessment to determine exposure to Leptospira sp. and distinguishes between vaccine and wild serovars. Thus, the presence or absence of vaccination in a patient's history is irrelevant, making this a useful in-house initial diagnostic test to identify leptospirosis. Early testing in France showed a 98 percent sensitivity and 93.5 percent specificity compared to diagnosis via microscopic agglutination test (MAT).¹⁵

Despite this, the ACVIM 2023 recommendations discuss that this test has a high specificity of over 97 percent in unvaccinated dogs with clinical presentations consistent with leptospirosis. However, they caution that the actual assay specificity and sensitivity will vary regionally with the circulating strains. They recommend MAT over the POC. They stress that a positive is highly likely but should be confirmed by MAT, while a negative does not rule out clinical disease. They recommend in negatives with a high index of suspicion, to follow up with another antibody test in one to two weeks.¹⁶

5) SNAP cPL ((qualitative) (Descriptive – positive/negative)). When considering a diagnosis of pancreatitis in a dog with clinical signs suggestive of disease, e.g. vomiting, lethargy, hyporexia, abdominal pain, or diarrhea, IDEXX's SNAP cPL reports results as either normal or abnormal. A normal suggests that pancreatitis is not likely to be the cause of clinical signs, while an abnormal (> 200 µg/L) value indicates that pancreatitis is "possible" (their words).¹⁷ However, it is known that cPL values can be > 200 µg/L with non-pancreatic disease conditions, including gastroenteritis, foreign bodies, primary liver disease, hyperadrenocorticism, kidney dysfunction, and more.^18–21

6) Spec cPL (quantitative) (Numerical, calculated/measured). IDEXX's Spec cPL SNAP test reportedly has a > 95 percent correlation between abnormal and normal results. The spec cPL test, unlike the original SNAP cPL, provides a measurable value from 0 to > 400. Those < 200 are considered normal (unlikely to have pancreatitis), those in the grey zone fall between 200-400, and those > 400 have a reported 78 percent specific and 93 percent sensitive evaluation for acute pancreatitis.¹⁷

SNAP and Spec cPL: Not as good as we hoped

Regardless of which cPL one utilizes, it should not be the sole basis for a diagnosis of pancreatitis.² With both of the available tests, a negative makes pancreatitis less likely. However, they may still miss early cases that have yet to trigger bloodwork changes. False positives occur too frequently to permit either test to be the sole means of diagnosis for acute pancreatitis in dogs.²²

Additional POCTs, including coagulation testing, T4, and blood gasses (e.g. NOVA, I-stat, or Epoc), may be valuable in emergency and critical cases.

POCTs: Yay or nay?

To be confident in results, routine tests, such as standard in-house heartworm tests, should be individually evaluated for the practice, its needs, sensitivity, specificity, and positive and negative predictive values.²³ In the face of possible clinical disease, clinical signs should be correlated with POCT results and confirmed with laboratory tests to ensure accurate diagnoses.

Factors in selecting a POCT include clinic purpose and need, incidence of cases and clinical indices of suspicion, accessibility to outside labs, timely access to additional diagnostic results, and patient load. Emergency, specialty, and 24/7 practices rely on in-house testing for immediate and key results to guide therapy. However, any clinic regularly seeing emergencies will also likely benefit from equipment such as in-house CBC, chemistry, and blood gas machines for rapid information. The sensitivity, specificity, and positive and negative predictive values of the tests selected should also be considered. False information and misdiagnosis based solely on a POCT can lead to a delay in care, undue suffering, and even unnecessary care.

Erica Tramuta-Drobnis, VMD, MPH, CPH, is the CEO and founder of ELTD One Health Consulting, LLC . Dr. Tramuta-Drobnis works as a public health professional, emergency veterinarian, freelance writer, consultant, and researcher. She is a member of the Evidence-Based Veterinary Medical Association (EBVMA), with different members writing this column. While all articles are reviewed for content, the opinions and conclusions of the author(s) do not necessarily reflect the views of the EBVMA or Veterinary Practice News. For information about the association or to join, visit the EBVMA website. 

References

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