Additionally, the sand usually "sparkles" on ultrasound if it moves. Sand is best diagnosed using a 3.5 megahertz probe. Horses with gastrointestinal rupture will have peritoneal fluid accumulation, sometimes with debris, visible on ultrasound. Horses with peritonitis will often have anechoic fluid, or material in between visceral surfaces. Differentiation between proximal enteritis and small intestinal obstruction is important to ensure correct treatment, and can be assisted with the help of ultrasound. Horses with small intestinal obstruction will usually have an intestinal diameter of -10 cm with a wall thickness of 3-5mm. Horses with proximal enteritis usually have an intestinal diameter that is narrower, but wall thickness is often greater than 6mm, containing a hyperechoic or anechoic fluid, with normal, increased, or decreased peristalsis. However, obstructions that have been present for some time may present with thickened walls and distention of the intestine. Horses experiencing intussusception may have a characteristic "bullseye" appearance of intestine on ultrasound, which is thickened, and distended intestine proximal to the affected area. Those experiencing nephrosplenic entrapment will often have ultrasonographic changes including an inability to see the left kidney and/or tail of the spleen. Abdominocentesis, or the extraction of fluid from the peritoneum, can be useful in assessing the state of the intestines. Normal peritoneal fluid is clear, straw-colored, and of serous consistency, with a total nucleated cell count of less than 5000 cells/microliter (24–60% which are neutrophils) and a total protein of 2.5 g/dL. Abdominocentesis allows for the evaluation of red and white blood cells, hemoglobin concentration, protein levels, and lactate levels. A high lactate in abdominal fluid suggests intestinal death and necrosis, usually due to strangulating lesion, and often indicates the need for surgical intervention. A strangulating lesion may produce high levels of red blood cells, and a serosanguinous fluid containing blood and serum. White blood cell levels may increase if there is death of intestine that leads to leakage of intestinal contents, which includes high levels of bacteria, and a neutrophil to monocyte ratio greater than or equal to 90% is suggestive of a need for surgery. "High" nucleated cell counts (15,000–800,000 cells/microliter depending on the disease present) occur with horses with peritonitis or abdominal abscesses.