Q&A column

Editor: Frederick L. Kiechle, MD, PhD

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Q. Should peritoneal dialysis fluid collected directly from a patient be considered peritoneal fluid or peritoneal dialysate fluid? A clinician at my institution placed an order for peritoneal dialysate fluid because the fluid was to be collected from the patient, not from the bag.

A.May 2022—The peritoneal cavity refers to a potential space, lined by a single layer of mesothelial cells, within the abdomen. It consists of the parietal peritoneum, which covers the abdominal wall and the diaphragm, and the visceral peritoneum, which covers such intra-abdominal organs as the liver, spleen, stomach, and intestines. With a surface area of 1 to 2 m², the peritoneal cavity is considered the largest serosal cavity in the body.¹ Under normal conditions, there is a small amount of lubricating fluid within the space that allows organs to move freely within the cavity.

Ascites is the accumulation of fluid within the peritoneal cavity due to pathological causes and most often results from liver cirrhosis. Other causes include malignancy, heart failure, and tuberculosis.² The clinical features of ascites include abdominal distension, abdominal discomfort, dyspnea, and weight gain. The principle physical exam finding is flank dullness. Performing maneuvers such as the fluid wave test and the shifting dullness test can increase the diagnostic accuracy of the physical exam. Imaging modalities, such as ultrasound, can be used to confirm ascites.

Once ascites has been diagnosed, the next step is usually to determine the underlying cause. This process frequently involves obtaining a sample of the fluid for laboratory analysis. Cell counts, special stains, adenosine deaminase activity tests, and cultures may be ordered in the infectious disease workup, whereas cytologic examination and tumor marker assays (e.g. CA125 and CEA) can be useful in the malignancy workup.³

Portal hypertension is a manifestation of liver cirrhosis, and the serum-ascites albumin gradient, or SAAG, is perhaps the most widely employed test when portal hypertension is suspected. The SAAG is calculated by subtracting the ascitic fluid albumin value from the serum albumin value obtained on the same day. Studies have shown that a SAAG result of ≥ 1.1 g/dL can identify portal hypertension 96.7 percent of the time.⁴

A nonpathologic cause of excess fluid in the abdomen is peritoneal dialysis. Patients with end-stage renal disease who meet certain requirements and prefer less disruption to their daily activities may choose peritoneal dialysis over hemodialysis. There are two types of peritoneal dialysis: continuous ambulatory peritoneal dialysis (CAPD) and automated peritoneal dialysis (APD).

With CAPD, dialysis fluid is instilled or drained manually at various times throughout the day. Automated peritoneal dialysis involves a device that performs these fluid exchanges typically while a patient sleeps at night.⁵ The dialysis solution, also known as dialysate, contains osmotic agents (e.g. glucose polymers), buffers (e.g. lactate and bicarbonate), and electrolytes (e.g. sodium, potassium, and magnesium). The dialysate is infused into the peritoneal cavity and allowed to dwell for a prescribed period of time, during which uremic toxins are eliminated from the circulatory system through the peritoneal membrane.