Diagnosing the Coughing Dog or Cat: Choosing the Right Diagnostic Test

A coughing dog or cat does not automatically need every available respiratory test. The more useful question is: Which diagnostic test will answer the clinical question raised by the history and examination? 

A practical work-up should progress from less invasive evaluation toward targeted imaging and airway testing. Importantly, a patient with respiratory distress should be stabilized before diagnostic procedures are performed. 

Start With Baseline Evaluation 

Before pursuing advanced diagnostics, baseline laboratory testing can help assess overall health, particularly when sedation or general anesthesia may be required. 

A complete blood count, serum biochemical profile, urinalysis, and heartworm disease testing are recommended as part of the initial evaluation^[1]. 

Thoracic imaging is then central to investigating the underlying cause of cough. Thoracic radiography is generally the first imaging test, allowing assessment of the trachea, pulmonary vessels, lung parenchyma, bronchial tree, and pleural space^[1],[2]. 

Three thoracic views, ventrodorsal, right lateral, and left lateral—are considered essential for a complete evaluation of the pulmonary parenchyma and can help identify lesions that may contribute to coughing^[3]. 

When Plain Radiographs Do Not Answer the Question 

Radiographs may suggest tracheal collapse, but they can underestimate its extent or fail to identify collapse at the carina^[1]. 

If dynamic airway collapse remains suspected despite inconclusive radiographs, fluoroscopy can provide additional information. It is particularly useful because cervical tracheal collapse occurs during inspiration, while intrathoracic tracheal collapse occurs during expiration^[1]. 

This distinction matters clinically. In one report, 8% of tracheal collapse cases diagnosed with fluoroscopy were not detected by radiography. Furthermore, the location of collapse identified radiographically was incorrect in 44% of cases subsequently examined with fluoroscopy^[1]. 

Where Does Ultrasound Fit? 

Thoracic ultrasound is particularly useful when radiographs identify abnormalities such as pleural effusion, pulmonary infiltrates, cranial mediastinal masses, or solitary pulmonary masses. It can also guide fine-needle aspiration or needle biopsy for cytology, histology, or culture. 

However, ultrasound does not provide a complete examination of the aerated thoracic cavity. Air and bone interfere with ultrasound transmission, while lesions surrounded by aerated lungs may not be visualized^[1]. 

Therefore, a normal or limited ultrasound examination should be interpreted in the context of the radiographic findings and clinical questions. 

When is CT Worth Considering? 

Thoracic CT provides greater contrast resolution and eliminates the superimposition of structures encountered with radiography. It can provide additional details of the mediastinum, airways, pulmonary parenchyma, and pleural space when radiographs are inconclusive or nonspecific^[2],[4]. 

CT can also identify foreign-body migration tracts, provide greater airway detail, and help characterize pulmonary masses and thoracic lymph nodes^[1],[2],[4]. 

Its value becomes particularly relevant when pulmonary nodules or metastatic disease are a concern. Compared with radiographs, CT can identify smaller pulmonary nodules. One reported comparison found that nodules needed to be approximately 7–9 mm to be detected radiographically but could be detected at approximately 1 mm with CT^[5]. 

The practical limitation is equally important: CT requires heavy sedation or anesthesia, so the patient's stability must be considered before proceeding. 

When Should Cardiac Testing Enter the Work-Up? 

When cardiac and respiratory causes remain difficult to distinguish, NT-proBNP can provide supportive information, particularly when other diagnostic findings are ambiguous^[1],[6]. 

In dogs with respiratory abnormalities, NT-proBNP concentrations below 800 pmol/L make respiratory disease more likely, whereas concentrations above 1,400 pmol/L increase the probability of congestive heart failure. In cats with respiratory clinical signs, concentrations above 270 pmol/L are likely associated with congestive heart failure^[1]. 

These values should not be interpreted in isolation. Pulmonary hypertension, systemic hypertension, renal dysfunction, sepsis, and sample handling can affect NT-proBNP results^[1]. Echocardiography can further evaluate cardiac changes consistent with congestive heart failure, although thoracic imaging remains necessary for definitive assessment of pulmonary findings. 

Clinical Bottom Line 

The diagnostic pathway for cough should be question-driven rather than test-driven. Start with baseline assessment and thoracic radiography, then select fluoroscopy, ultrasound, CT, cardiac testing, or other investigations according to what remains unresolved. This approach can provide more useful information while avoiding unnecessary invasive procedures.

References:

1. Hsieh BM, Beets AK. Coughing in small animal patients. Frontiers in Veterinary Science. 2020 Jan 21;6:513. https://www.frontiersin.org/journals/veterinary-science/articles/10.3389/fvets.2019.00513/pdf
2. Prather AB, Berry CR, Thrall DE. Use of radiography in combination with computed tomography for the assessment of noncardiac thoracic disease in the dog and cat. Veterinary Radiology & Ultrasound. 2005 Mar;46(2):114-21. https://doi.org/10.1111/j.1740-8261.2005.00023.xDigital Object Identifier (DOI)
3. Armbrust LJ, Biller DS, Bamford A, Chun R, Garrett LD, Sanderson MW. Comparison of three-view thoracic radiography and computed tomography for detection of pulmonary nodules in dogs with neoplasia. Journal of the American Veterinary Medical Association. 2012 May 1;240(9):1088-94. https://krex.k-state.edu/bitstreams/fd1590d2-3008-4c67-a25f-c18422d48cfc/download
4. Dear JD. Bacterial pneumonia in dogs and cats. The Veterinary Clinics of North America. Small Animal Practice. 2013 Nov 21;44(1):143. https://pmc.ncbi.nlm.nih.gov/articles/PMC7114586/pdf/main.pdf
5. Nemanic S, London CA, Wisner ER. Comparison of thoracic radiographs and single breath-hold helical CT for detection of pulmonary nodules in dogs with metastatic neoplasia. Journal of veterinary internal medicine. 2006 May;20(3):508-15. https://onlinelibrary.wiley.com/doi/pdfdirect/10.1111/j.1939-1676.2006.tb02889.x
6. Oyama MA, Rush JE, Rozanski EA, Fox PR, Reynolds CA, Gordon SG, Bulmer BJ, Lefbom BK, Brown BA, Lehmkuhl LB, Prosek R. Assessment of serum N-terminal pro-B-type natriuretic peptide concentration for differentiation of congestive heart failure from primary respiratory tract disease as the cause of respiratory signs in dogs. Journal of the American Veterinary Medical Association. 2009 Dec 1;235(11):1319-25. https://www.holisticandorganixpetshoppe.com/uploads/7/8/8/4/7884424/javma200912-assessment-ntprobnp.pdf