Laboratory Evaluation of Pericardial Effusion

What do you do with the fluid *after* you've saved the patient?

Echocardiogram from a dog with pericardial effusion and a right-sided heart mass

OK, so you have a patient with pericardial effusion, and you’ve stabilized them in the short term with pericardiocentesis. Great job! Now what do you do? This article will walk you through how to process and evaluate the fluid you’ve removed, and what it may (or may not) tell you.

Preliminary Analysis

The first step to any fluid is to look at it grossly and note the appearance: Does it look like frank blood or serosanguinous? Thin and straw-colored? Thick and cloudy? These, in combination with other data, can point towards distinct differentials. Next, check a PCV and total solids (TS) on the fluid, and put some of the fluid in a red top tube (no anticoagulant). Wait a few minutes to see if it clots or not; iatrogenic hemorrhage from nicking a vessel during the procedure should rapidly coagulate like normal blood, while a long-standing effusion, even if grossly very bloody, should not clot.

Any fluid with a PCV >10% is considered a hemorrhagic effusion, which is the most common cause of pericardial effusion. Recent hemorrhage is more likely to have a PCV close to peripheral blood, while chronic, insidious hemorrhage will be quite a bit lower. The TS data can help classify non-hemorrhagic effusions, and we’ll get to that in a minute.

The last core component of fluid analysis is assessing the total nucleated cell count (TNCC). This can be done by a semi-quantitative estimate from a direct fluid smear or by running the effusion through a CBC analyzer. Many—but not all!—analyzers can process cavitary effusion samples; check with your manufacturer instructions and documents.

Effusion Classifications

Pericardial effusions can be broken down into several distinct groups of diseases by mechanism as follows:

Hemorrhagic

• Characteristics: >10% PCV, non-clotting blood, supported by cytology

• Causes: ***Neoplasia*** (most common), idiopathic (second most common, about 20% of cases), coagulopathy, trauma, left atrial rupture (very rare)

Exudate

• Characteristics: TNCC >5,000/uL, TS >2.5 g/dL

• Causes: Neoplasia, septic inflammation, sterile inflammation (i.e. 2° to chronic hemorrhage, neoplasia, chyle, others)

Protein-poor (“pure”) transudate

• Characteristics: TNCC <5,000/uL (usually MUCH lower), TS <2.5 g/dL

• Causes: Increased hydrostatic pressure, decreased plasma oncotic pressure (severe hypoproteinemia)

Protein-rich (“modified”) transudate

• Characteristics: TNCC <5,000/uL (usually MUCH lower), TS >2.5 g/dL

• Causes: Vasculitis, vascular/lymphatic obstruction, heart failure (rare in dogs)

Cytology

Cytology can support the classifications above and may provide further information, i.e. identifying neoplastic cells or infectious organisms. It is always recommended to make direct or straight fluid slide(s), and further concentration by centrifugation and/or cytospin techniques (including homemade versions)¹ may increase cellular yield for rare findings. As stated above, pericardial effusions are often very bloody. Several cytology findings can help discriminate peracute or iatrogenic hemorrhagic from chronic hemorrhage.