Body fluid cytology (effusion cells) in cats and dogs
When fluid collects in the chest, abdomen or around the heart, a sample is spread on slides and the cells are examined under the microscope. The type of cells — neutrophils, lymphocytes, macrophages, tumour cells, bacteria inside cells — adds meaning to the protein and cell count. It is a strong clue for the vet, but not always a final answer.
What it shows
In healthy dogs and cats the chest, abdomen and sac around the heart hold very little fluid — generally less than 10 mL — and it normally cannot be drawn off. When fluid builds up, the vet takes a sample, and besides measuring protein and counting cells (effusion analysis), the laboratory spreads the fluid on slides and looks at which cells are there.
The cytologist describes the main cell types — neutrophils (and whether they look damaged, “degenerate”), macrophages, lymphocytes, the cells lining the cavity (mesothelial cells), red cells — and anything that does not belong: bacteria, food material, bile, crystals or tumour cells. The result is a description and a conclusion, not a number.
Higher than reference — what it may be a sign of
- Mostly neutrophils — an exudate, that is, inflammation. Causes include bacteria, fungi, parasites, a ruptured organ, tumours, urine and bile. Degenerate neutrophils point especially to bacteria; bacteria seen inside neutrophils or macrophages support a septic (infected) effusion, and a mixed population of gut bacteria with food material supports gut rupture.
- Mostly small lymphocytes, with fat-laden macrophages, in white to pink opaque fluid — the typical picture of a chylous effusion; confirmed by triglycerides in the fluid.
- Yellow-brown pigment, free or inside macrophages — may be bile; it can be hard to tell from haemosiderin, a breakdown product of red cells.
- Sperm or urine crystals in abdominal fluid — support urine in the abdomen, together with creatinine and potassium of the fluid.
- Tumour cells — lymphoma, mast cell tumours, plasma cell tumours, acute leukaemia and carcinomas shed cells into fluid most readily. Mesothelioma is rare.
- Haemosiderin inside macrophages — it takes more than 24 hours to form, so it means bleeding has lasted for some time. Red cells being eaten by macrophages mean more on a fresh smear, because in stored fluid this also happens in the tube.
Lower than reference
Cytology does not always find the cause. A smear without tumour cells does not rule out a tumour. Reactive mesothelial cells can be large, multinucleated and look malignant, and it can be very difficult to tell them from carcinoma or mesothelioma cells. The cause is then sought with imaging, other tests and biopsy.
Look at together with
- Protein and cell count of the fluid — the first sort into transudate and exudate; cytology explains what lies behind the numbers.
- Lactate and glucose of the fluid — measured when septic peritonitis is suspected.
- Bacterial culture — of fluid collected in a sterile container when infection is suspected.
- Rivalta test — in a cat with suspected FIP.
- Imaging — ultrasound of the heart is the most sensitive test for fluid around the heart and shows a tumour in most cases where one is the cause.
Units and conversion
There are no units: the result is the cytologist’s description and conclusion — for example “neutrophilic inflammation with intracellular bacteria” or “no atypical cells seen”. In the app the result is kept as text, the way it is written on the form.
Reference range
The reference range is on your lab’s form — for cytology it is not a number but a description. Normal fluid contains mainly macrophages and intact neutrophils, small numbers of small lymphocytes and some mesothelial cells. Fresh smears matter: in stored fluid cells deteriorate, neutrophils swell and look degenerate, macrophages start eating red cells and bacteria within 30–120 minutes, and bacteria may overgrow. That is why fluid goes into an EDTA (purple-top) tube and smears are made soon after collection.
Cats and dogs
In cats with FIP the fluid is typically viscous and yellow with fibrin strands, rich in protein but with relatively few cells — mostly mildly degenerate neutrophils and macrophages. Chylous fluid in the chest of cats can be linked with heart disease.
In dogs, bleeding into the sac around the heart is associated with idiopathic pericarditis and haemangiosarcoma; a smear without tumour cells does not rule out a tumour. A benign ruptured splenic haematoma causing blood in the abdomen is seen mostly in dogs, and so is bile leakage from a ruptured gallbladder mucocele. In male dogs, sperm in abdominal fluid supports urine leakage.
Add a lab form — Pet Tracker Space recognises the lines, keeps the form’s units and draws each indicator over time with your lab’s reference band.
Sources
- eClinPath (Cornell University) — Effusions
- eClinPath (Cornell University) — Peritoneal fluid
- eClinPath (Cornell University) — Cytology: sample collection
- Merck Veterinary Manual — Cytology (diagnostic procedures for the private practice laboratory)
- Merck Veterinary Manual — Pericardial disease in dogs and cats
This page explains what a lab value means. It is not a diagnosis and not a substitute for your veterinarian — only they can interpret results for your pet.