Triglycerides and cholesterol in effusion fluid (chylous effusion) in cats and dogs

What it shows

Chyle is lymph from the intestines that carries absorbed dietary fat in tiny particles (chylomicrons), which are rich in triglycerides. It normally flows through the thoracic duct into the bloodstream. When chyle leaks into the chest or abdomen, the result is a chylous effusion — in the chest it is called chylothorax. The fluid usually looks white to pink and opaque and may form a cream layer on standing. Two lines on a form help confirm it:

• Effusion triglycerides — the main marker. It is measured on a chemistry analyser, ideally together with blood triglycerides from a sample taken at the same time.
• Effusion cholesterol — used less often. With triglycerides it gives a cholesterol-to-triglyceride ratio, which helps tell a true chylous effusion from a “pseudochylous” one — a milky-looking fluid without chyle, which is quite rare in animals.

Higher than reference — what it may be a sign of

• Triglycerides higher in the fluid than in the blood — this is the hallmark of a chylous effusion. Fluid triglycerides are usually above 100 mg/dL and usually more than twice the serum value; because the vast majority of chylous effusions exceed 100 mg/dL, a blood value is not always needed.
• Cholesterol-to-triglyceride ratio below 1 in the fluid also supports a chylous effusion.
• What the microscope adds — the cell count is usually above 3,000/µL, and small lymphocytes predominate; macrophages or neutrophils may contain fat droplets.

A chylous effusion is a finding, not a cause. Pressure on the thoracic duct or veins can come from heart failure, heartworm infection, fungal disease or tumours in the chest; injury to the chest is another cause. When no cause is found after a full work-up, the condition is called idiopathic. The vet decides what to look for next.

Lower than reference

Triglycerides that are not raised in the fluid make a chylous effusion less likely, and the fluid is then classified by its protein and cells instead. One caveat matters: in an animal that has not been eating, the chyle carries little fat, so the fluid may not look milky and triglycerides may not be clearly high; the cholesterol ratio may not help either. In such cases the vet relies more on the cells under the microscope.

Look at together with

• Blood triglycerides and cholesterol — taken at the same time, for comparison with the fluid.
• Effusion analysis — protein and cell count; in chylous fluid the refractometer protein may be inaccurate, because fat particles change how light bends.
• Lymphocytes — a fluid dominated by small lymphocytes fits chyle.
• Tests for the cause — in cats, blood tests for FeLV, FIV and heartworm, heart ultrasound and imaging of the chest.

Units and conversion

Triglycerides and cholesterol in fluid use the same units as in blood: mmol/L in most European labs, mg/dL in North America. For triglycerides 1 mg/dL = 0.01129 mmol/L, so the often-quoted 100 mg/dL is about 1.13 mmol/L. For cholesterol 1 mg/dL = 0.02586 mmol/L, so 100 mg/dL is about 2.59 mmol/L. Because the two factors differ, the cholesterol-to-triglyceride ratio is calculated with both values in the same unit and comes out different in mg/dL and mmol/L. In the app each row keeps the unit printed on the form.

Reference range

The reference range is on your lab’s form; for fluid many labs print no interval at all, because the fluid is interpreted against the blood value and against cut-offs for chylous effusion rather than against a “normal” range. The comparison with a blood sample taken at the same time is often the most telling part of the result.

Cats and dogs

Sources

1. eClinPath (Cornell University) — Effusions
2. eClinPath (Cornell University) — Peritoneal fluid
3. Merck Veterinary Manual — Diagnostic techniques for respiratory disease in animals
4. VCA Animal Hospitals — Chylothorax in cats
5. Merck Veterinary Manual (Dog Owners) — Accumulation of fluid or air in the chest cavity in dogs