Colic - Peritoneal Fluid Analysis

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Assessing the pertoneal fluid can give information about the disease process in the very early stages. The fluid sample is collected by abdominocentesis, and it should be used in the work-up of severe, recurrent or persistent colic cases. The results may indicate whether surgery is required and potentially provide a diagnosis and prognosis.

Some disease processes such as peritonitis and enteritis produce changes in the peritoneal fluid before the onset of clinical signs. This allows for the early diagnosis of the disease which will favour an improved prognosis for the patient. Strangulating obstructions produce changes in the peritoneal fluid within a few hours of the obstruction occurring. Horses with an intussusception may have normal peritoneal fluid initially as the peritonitis that develops is localised to the portion of entraped bowel.


Peritoneal fluid should be assessed for colour, turbidity, specific gravity, total protein, total nucleated cell count, differential cell count and examined under the microscope for cytology. The cytology sample should be stained with Wright's and/or Gram stain and examined for the presence of bacteria (intracellular or extracellular), plant material, cellular appearance, a white blood cell count and a differential cell count. Glucose and lactate levels can also be measured.

Normal Peritoneal Fluid

  • Clear to yellow in colour
  • Specific gravity of 1.005 mg/dl
  • Total nucleated cell count of less than 10 (x 109L)
  • Total protein of less than 25g/L

Normal peritoneal fluid is a filtrate of the blood secreted by the peritoneum and it acts to lubricate the abdominal organs, preventing infection and the formation of adhesions. It is drained by specialist lymphatic lacunae in the diaphragm.

Abnormal Peritoneal Fluid

The peritoneal fluid may be a transudate, modified transudate or exudate, depending on the level of total protein and the total nucleated cell count. An exudate is formed in the presence of peritoneal inflammation and ischaemia (aka severe colic), due to an increase in capillary wall permeability. Inflammatory cells such as neutrophils, macrophages and lymphocytes pass from the capillaries into the peritoneum, producing a fluid with a high total protein and total nucleated cell count. Volume should increase over time, but it may be pocketed within the abdomen so too much significance should not be placed on this. Hyperbilirubinaemia is common in anorexic horses, giving the fluid a very yellow appearance.

Visual Inspection

The initial changes seen in the fluid during gastrointestinal disease show a serosanguinous appearance. The fluid becomes yellow or white and increasingly turbid when there is more protein or cells in it. Red, brown or green fluid can indicate rupture of the gastrointestinal tract and plant material may be present in the sample. In cases of haemorrhage the fluid should not clot, unless it is very acute. If the gut is distended but no ischaemia is present then peritoneal fluid may be completely normal, or have a slightly high volume, total protein and nucleated cell count level. Peritonitis cases where the bowel is not ischaemic have a large buffy coat when left to settle.

It is important to remember that a blood tinged sample may be the result of iatrogenic contamination. This can be differentiated by spinning the sample down, a pellet of red blood cells will form in iatrogenically contaminated samples - in compromised samples it will not. Equally, if plant material is present it is important to ensure that the bowel was not directly sampled. A dark red sample may indicate accidental splenic or vessel puncture. The sample will have a higher Packed Cell Volume (PCV) than the peripheral blood if it has been obtained from the spleen, and the same PCV as the peripheral blood if it has come from a vessel.


The sample should be smelt. The presence of urine, bacteria or gut content may be detectable.

Total Protein and Specific Gravity

Total protein and spoecific gravity should be analysed using a refractometer. Levels should be increased where peritonitis, severe inflammation and ischaemia are present. Levels should be normal in cases of medical colic.

Total Nucleated Cell Count

Total nucleated cell count should correlate fairly closely with the TP and specific gravity, with levels increasing in cases of peritonitis, ischaemia and inflammation. Levels should remain normal in medical colics.


Cytology is not always performed in the emergency colic work-up. Inflammatory cells, red blood cells and mesothelial cells (the cells that line the peritoneum) should be examined and counted. The sample should also be examined for bacteria and plant material. A high nucleated cell count suggests a ruptured bowel but this is not always the case - sometimes the nucleated cell count is normal because the cells may have been lysed in the fluid. Cases of septic peritonitis will have samples with predominantly toxic and degerate neutrophils. Bacteria may be visible in septic peritonitis or rupture of the gut.

Bacterial Culture

Baterial culture should be performed when peritonitis is suspected. Its use is not appropriate when deciding surgical or medical treatment in emergency colic case, as results are not immediately available.

Other Tests

The fluid can be assessed for glucose and lactate level. A sample with a glucose concentration higher than that of the peripheral blood indicates septic peritonitis. If the peritoneal lactate concentration is higher than the peripheral blood concentration then intestinal infarction is likely. Serum and peritoneal fluid creatinine levels can be measure in cases where uroperitoneum is suspected (peritoneal creatinine levels should be double than of plasma in these cases).

Summary of Peritoneal Fluid Parameters and Intra-abdominal Disorders

Clinical Condition Appearance Total Protein (g/dl) Total Nucleated Cells/L Cytology
Normal Clear - Yellow < 2.5 < 10 x109 20 - 80% mononuclear cells, 40 - 80% neutrophils
Non-strangulating Obstruction Clear - Yellow, Slightly turbid < 3.0 < 3.0 - 15.0 x 109 Mostly neutrophils (preserved)
Strangulating Obstruction Red - Brown, Turbid 2.5 - 6.0 5.0 - 50.0 x 109 Mostly neutrophils (degenerate)
Proximal Enteritis Yellow - Red, Turbid 3.0 - 4.5 < 10.0 x 109 Mostly neutrophils (preserved)
Bowel Rupture Red - Brown - Green, Turbid with or without particulate matter 5.0 - 6.5 20 - 150 x 109 > 95% neutrophils ( severely degenerate), intracellular or extracellular bacteria, with or without particulate matter
Septic Peritonitis Yellow - White, Turbid > 3.0 20.0 - 100.0 x 109 Mostly neutrophils (degenerate)
Accidental Enterocentesis Brown - Green, with or without paticulate matter Variable < 1.0 x 109 Free bacteria, plant material, few cells
Intraabdominal haemorrhage Dark red Similar TPP to peripheral blood White Blood Cell Count increases with time PCV less than peripheral blood, erythrocytophagia, few if any platelets
Post-celiotomy Yellow - Red, Turbid Variable Variable Mostly neutrophils (moderate degeneration), no intracellular bacteria

Abdominocentesis and peritoneal fluid analysis are important tools in the colic work-up. They provide information on diagnosis and prognosis, and can help the clinician decide whether or not the colic case will require surgical treatment.

Colic - Peritoneal Fluid Analysis Learning Resources
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Cytology Q&A 15


Edwards B. (2009) Diagnosis and Pathophysiology of Intestinal Obstruction, in Equine Gastroenterology courtesy of the University of Liverpool, pp 8 - 9

Meuller E, Moore J. N, (2008) Classification and Pathophysiology of Colic, Gastrointestinal Emergencies and Other Causes of Colic, in Equine Emergencies- Treatments and Procedures (3rd Edition) Eds Orsini J. A, Divers T.J, Saunders Elsevier

Milne, E (2004) Peritoneal fluid analysis for the differentiation of medical and surgical colic in horses In Practice 2004 26: 444-44

Rose R.J, Hodgson D.R (2000) Examination of the Alimentary Tract, Alimentary Tract, Manual of Equine Practice, 2nd Edition, Saunders Elsevier, pp 284 - 286

RVC staff (2009) Alimentary System RVC Intergrated BVetMed Course, Royal Veterinary College

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