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Fluid analysis in veterinary medicine: Effusion Confusion | VETgirl Veterinary Continuing Education Podcasts

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Normally, our small animal veterinary patients have a very small amount of fluid within their body cavities. We can not see this radiographically, and most novice users of the ultrasound machine would also likely miss this effusion. The main goal of this fluid is to lubricate the surfaces of the organs and body walls like motor oil for your car engine. This allows the organs to glide over each other without friction, avoiding inflammation. That is in health. However, in states of disease, we see effusion develop which needs to be identified and characterized for both diagnosis and targeted treatment. So, if you see a dog or cat with abdominal effusion or pleural effusion, rapid fluid analysis is imperative!

Effusions are generally characterized into one of 3 categories:

– Transudate
– Modified Transudate
– Exudate

Transudates
Transudates are often found because of either increased hydrostatic pressure or decreased colloid osmotic pressure (COP). Either of these situations will alter fluid balance resulting in effusion. By definition, transudate fluids are often low protein (< 2.5 g/dL) and low or absent regarding cellularity (nucleated cell count < 2500/l. Common causes of transudative effusions include congestive heart failure, liver failure, nephrotic syndrome, protein losing diseases (e.g., protein losing enteropathy, commonly seen in Yorkies or protein losing nephropathy, often seen in Labrador retrievers and Golden retrievers), and in some cases of neoplasia.

Modified Transudates
Modified transudates are identified when there is a mild increase in both total protein (typically 3.0-5.0 g/dl) and nucleated cell count (>2500/μl and less than 5000/μl). Modified transudates are commonly seen when a transudate has been chronic, resulting in an inflammatory reaction. As compared to the often-clear gross appearance of a transudate, a modified transudate may appear cloudy in appearance. Common causes of modified transudates include increased hydrostatic pressure, right-sided congestive heart failure, left-sided congestive heart failure in cats, decreased colloid osmotic pressure (i.e. hypoalbuminemia), lymphatic obstruction, and neoplasia.

Exudates
Exudates are identified when there is an abnormally high total protein and nucleated cell count. Total proteins range between 3.0 and 7.0 g/dl and total cell counts are typically greater than 5000/μl. Common causes of exudates include inflammation, infection (e.g., pyothorax), hemorrhage, chlye, and neoplasia.  While classifying the effusion based on cell count and protein level may seem tedious, it can be quite helpful in developing a differential list and therefore a more specific diagnostic and treatment plan.

Fluid collection and further evaluation
When abnormal fluid (effusion) is suspected, either a blind centesis procedure can be performed (without ultrasound) or an ultrasound-guided centesis (thoracocentsis or abdominocentesis) can performed. Focused assessment using sonography for triage, tracking, and trauma (FAST3) is a newly describe technique for rapid ultrasound technique to identify free fluid.

Once the fluid sample is collected, it is recommended to prepare the sample for either in-house evaluation of submission to the diagnostic laboratory for further evaluation. If the sample is being submitted to an outside diagnostic laboratory, fluid should be placed in an EDTA tube and red top tube. The EDTA collected sample can be submitted for RBC count, nucleated cell counts, cytology, or other potential tests including flow cytometry and PCR testing. A red top tube can be saved for other diagnostics (e.g.., aerobic and anaerobic bacterial culture, mycoplasma, and fungal cultures, total protein, albumin, bilirubin, creatinine, potassium, triglyceride, glucose, lactate, and lipase). The author also prefers to use some of the fluid to prepare a slide for evaluation as delay in fluid sample submission may lead to artifactual changes in cell morphology when in the sample tubes.

While certain disease conditions may permit delay in diagnostic results, there are conditions which should be evaluated on a more emergent, in-house basis.

Septic Effusion
Septic effusion, notably a septic abdominal effusion, is typically considered a surgical emergency and therefore delay in sample submission and evaluation may increase both morbidity and mortality. In other words, look at this in-house immediately. Septic abdominal effusions can be seen as a result of a ruptured gastrointestinal tract due to causes including trauma and neoplasia. When evaluating a septic effusion, in-house cytology will often demonstrate a markedly suppurative effusion with abundant neutrophils as well as bacterial, typically intracellular bacteria. While the author uses cytology as the gold standard in-house test for immediate patient assessment, supportive diagnostic testing may include comparison of abdominal fluid and peripheral blood lactate and glucose concentrations. When using lactate as a clinical tool, a septic patient will have an abdominal fluid lactate 2X the level of the peripheral blood lactate on a paired sample. When comparing the patient’s glucose levels, a septic patient will have an abdominal fluid glucose concentration 20mg/dL lower than the paired peripheral glucose sample (NOTE: You can’t use your point-of-care glucometer like the AlphaTRAK 2 for this – you must run your abdominal effusion sample through your biochemistry analyzer for accurate results). Due to the disparity which may be seen in certain conditions such as severe suppurative disease processes, cytology remains the most reliable test.

Uroabdomen
The most useful evaluation of effusion for identification of a uroabdomen is creatinine, combined with potassium. Blood urea nitrogen (BUN) is not as helpful when evaluating for the presence of a uroabdomen, due to the size of the molecule (it rapidly dialyzes between the peritoneal membrane and the blood). A patient with a uroabdomen will have a creatinine level in the abdominal effusion that is 2X or greater than serum creatinine. Potassium concentrations consistent with a uroabdomen are greater than 1.4:1 (canine) and 1.9:1 (feline).

Bile Peritonitis
Patients with a bile peritonitis will often have a green-tinged fluid present, although this may be masked by hemorrhage or other processes based on the underlying disease process. Bilirubin crystals may be seen in the fluid as well. A patient with a bile peritonitis will have a bilirubin level in the abdominal effusion that is two or more times greater than serum bilirubin, although in the authors experience the effusion bilirubin is often significantly higher than in the serum.

When in doubt, take the time to look at that effusion in-house so you can help diagnose your veterinary patient appropriately and quickly!

References:
1.  Beal MW. Approach to the acute abdomen. Vet Clin Small Anim. 2005;35:375–396.
2.  Bischoff MG. Radiographic techniques and interpretation of the acute abdomen. Clin Tech Sm Anim Pract. 2003;18(1):7–19.
3.  Boysen SR, et al. Evaluation of a focused assessment with sonography for trauma protocol to detect free abdominal fluid in dogs involved in motor vehicle accidents. JAVMA. 2004;225(8):1198–1204.
4.  Bonczynski JJ, et al. Comparison of peritoneal fluid and peripheral blood pH, bicarbonate glucose and lactate concentration as a diagnostic tool for septic peritonitis in dogs and cats. Vet Surg. 2003;32(2):161–166.
5.  Culp WT, Holt DE. Septic peritonitis. Compendium. October 2010:E1–E15.
6.  Heeren V, et al. Acute abdomen: diagnosis. Compendium. May 2004:350–363.
7.  Heeren V, et al. Acute abdomen: treatment. Compendium. May 2004:366–373.
8.  Vinayak A, Krahwinkel DJ. Managing blunt trauma induced hemoperitoneum in dogs and cats. Compendium. April 2004:276–291.

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