Synonyms Abdominal Paracentesis; Ascites Fluid Tap

Procedure Commonly Includes At the bedside, physician introduces a needle into the peritoneal space of a patient with free ascites, and samples the fluid for diagnostic and/or therapeutic purposes.

Indications Diagnostic indications include:

patients with new onset of ascites

ascites fluid of unknown etiology

patients with clinically suspected ascites fluid infections (abdominal pain, unexplained fever, leukocytosis, declining mental status)

Therapeutic paracentesis is indicated when ascites fluid has accumulated enough to cause respiratory compromise, abdominal pain, or worsening of existing inguinal or umbilical hernias. Paracentesis should not be performed to diagnose the presence of ascites fluid. This should be known prior to the procedure (by physical examination or radiological imaging).

Contraindications Severe coagulopathy not correctable by vitamin K, fresh frozen plasma, etc; inability of physician to demonstrate ascites fluid on physical examination; lack of patient cooperation. Recent literature suggests the following factors are not contraindications for paracentesis: morbid obesity, low grade coagulopathy, multiple abdominal surgical scars, and bacteremia.¹

Patient Preparation Technique and risks of the procedure are explained. Premedications (eg, sedatives or narcotics) are not routinely required. Laboratory requisitions are completed in advance to avoid delay in fluid processing later. Prothrombin and partial thromboplastin times prior to paracentesis are ordered at physician discretion (some elect to transfuse fresh frozen plasma immediately prior to procedure if PT/PTT are prolonged).

Aftercare No special limitations exist for the patient postprocedure. If large amounts of ascites are removed (several liters), frequent blood pressure measurements are needed to monitor possible hypotension. Patients may ambulate postprocedure if vital signs remain stable. Occasionally, ascites fluid may leak persistently from the puncture site; in this instance, the patient should remain supine with the site angled directly upwards, until the leak stops spontaneously.

Special Instructions In clinical practice, paracentesis is at times performed on patients with significant hepatic encephalopathy. Additional personnel may be required for conferring with family members and for proper patient positioning during the procedure.

Complications The medical literature is divided on the incidence of complications from paracentesis. Earlier literature was more negative and tended to emphasize the possible complications, based on retrospective analysis.^2,3 Some authors suggested that paracentesis itself was the cause of many cases of ascites fluid infection.⁴ A recent prospective study concluded that paracentesis is a safe procedure, carrying <1% risk of major complications and <1% risk of minor complications.¹ No deaths or bowel perforations were seen in 229 consecutive attempts. The most feared complication is needle perforation of an abdominal viscus or solid organ such as liver or spleen. Others include: intraperitoneal hemorrhage from laceration of an umbilical vein, scrotal or penile edema, abdominal wall hematoma, contamination of ascites by nonsterile technique. Hypotension can be seen when large amounts of ascites (more than 1500 mL) are removed rapidly.

Equipment Sterile gloves, drapes (optional), and adequate local anesthesia (26-gauge subcutaneous needle, 2% lidocaine). In clinical practice, various needles and angiocatheters are used. A 22-gauge, 1.5" metal needle with a plastic catheter is recommended. If a thick panniculus is encountered, a 3" to 5" 22-gauge needle may be substituted. Also required are a sterile 50 mL syringe and, if large volumes of ascites are to be removed, a sterile 1 L vacuum bottles with connecting tubing.

Technique Patient empties bladder prior to procedure. Physician confirms presence of ascites by physical examination with patient in a semirecumbent position. Preferred site of entry is in the midline, inferior to the umbilicus. If a midline scar is present from prior surgery or if percussion is not reliable, an area near the flank is selected. At times, physician may request patient to assume the hand-knees position if small amounts of ascites are present. The entry site is then caudad to the umbilicus. The site is prepped with iodine solution and skin and deeper tissues are infiltrated with lidocaine. The skin is retracted caudally and the 22-gauge needle (attached to syringe) is inserted into the anesthetized area and advanced while aspirating. When ascites fluid returns freely, the needle is held in position and not advanced further (avoiding bowel trauma). Multiple aliquots (50 mL) may be obtained in this manner. For larger volumes, the syringe is removed and connecting tubing is directly attached to the 22-gauge needle to allow drainage into vacuum bottles. Once the desired amount is collected, the needle is withdrawn quickly and the caudal skin retraction is released, allowing the skin to return to its normal position. This causes the entrance and exit needle sites to form a "Z-tract" which minimizes ascites leakage.

Data Acquired Ascites fluid is routinely analyzed for cell count and differential, chemistries including LD, albumin and protein, Gram's stain, bacterial culture, and cytology. Additional tests include special cultures for tuberculosis or fungi, ascites fluid pH, amylase, lipase, glucose, triglycerides, lactate, CEA, and hyaluronic acid.

Specimen When the procedure is performed therapeutically, the maximum volume of ascites that can be removed safely depends on the presence or absence of peripheral edema.⁵ It is recommended that in patients without edema, the upper limit should be 1500 mL. Patients with peripheral edema may tolerate larger volumes without hypotension (in one study, 5 L).⁶ When performed for diagnostic purposes, smaller volumes (50-100 mL) are adequate for routine studies. If malignancy or fastidious infection is suspected, larger volumes (more than 100 mL) will improve laboratory yield.

Container Purple top tube for cell count; red top tube for routine chemistries; aerobic and anaerobic culture media bottles for bacteriology. For cytology, send sterile vacuum bottles with 5000 units of heparin added. If ascites fluid pH desired, send specimen in anaerobic syringe (gas bubbles removed) on ice to acute care laboratory.

Collection Some authorities recommend inoculating the bacterial culture media with ascites fluid immediately at bedside.⁷ The average concentration of bacteria in ascites fluid is very low in most cases of spontaneous peritonitis. In addition, a significant number of organisms may not survive in the time needed for specimen transport and plating in the Microbiology Laboratory. Bedside inoculation of appropriate media (standard blood culture bottles) may improve the chances of obtaining a positive bacterial culture several fold.

Normal Findings Ascites fluid is traditionally categorized as either "exudative" or "transudative" based on laboratory analysis.⁸ Transudative ascites is caused for the most part by cirrhosis physiology; that is, increased portal venous pressure or decreased portal venous colloid osmotic pressure. Examples of transudates include hepatic cirrhosis, congestive heart failure, constrictive pericarditis, Budd-Chiari syndrome, inferior vena caval obstruction, and nephrotic syndrome. Exudative ascites is generally noncirrhotic in its pathophysiology and may be due to peritoneal membrane permeability defects. Examples of exudates include malignancy, spontaneous bacterial peritonitis (SBP), or other ascites infections (such as tuberculosis), vasculitis, pancreatitis, myxedema.

Critical Values Transudates are characteristically "low-protein" ascites and have been defined by ascites protein <3 g/dL; exudates >3 g/dL. Exceptions are common and other laboratory tests are often used in conjunction with the protein concentration. These include (for transudates): LD <200 units/L, protein ascites/serum ratio <0.5, LD ascites/serum ratio <0.6. Values outside these ranges support the diagnosis of an exudate. The "albumin gradient," defined as serum albumin minus ascites albumin, has recently been shown to accurately identify ascites caused by portal hypertension physiology (eg, cirrhosis).⁹ An albumin gradient >1.1 is considered transudative and is due to an oncotic (albumin) pressure gradient between the systemic arterial pressure and ascites fluid, as seen with elevated portal pressures. Exudates tend to have gradients <1.1. The early diagnosis of spontaneous bacterial peritonitis (SBP) prior to bacterial culture results can frequently be made on routine analysis of ascites fluid.¹⁰ Patients with SBP, or other ascites fluid infections, have ascites WBC count >500/mm³ along with many polymorphonuclear (PMN) cells on the differential (>250/mm³). In addition, two other laboratory indices suggestive of SBP are ascites pH <7.35 and ascites lactate <25 ng/dL.¹¹ The clinical utility of these last two criteria has not been as well established as the standard PMN count. Many physicians will begin empiric antibiotics on the basis of PMN >250/mm³ alone. Gram's stain of ascites fluid has low sensitivity for detecting SBP due to the low bacterial concentration, even on a centrifuged sample. Malignant ascites can be expected to have abnormal cytology in >50% of the cases. Indirect evidence of neoplasm include: grossly hemorrhagic fluid (may also be traumatic); ascites CEA >10 ng/mL with adenocarcinoma; ascites hyaluronic acid >0.25 mg/mL with mesothelioma; high ascites triglyceride levels with chronic chylous ascites (>80% of cases are lymphoma); ascites WBC count >500/mm³ with peritoneal carcinomatosis (but PMN count low, <250/mm³), pH <7.35, lactate <25 mg/dL. None of these values are considered diagnostic of malignancy and should be used only as supportive evidence.

Limitations As described previously, the strict use of the ascites protein concentration alone in differentiating exudate from transudate has considerable potential for error. Multiple criteria should be considered, including the albumin gradient and relevant clinical findings.

Additional Information Paracentesis is a safe procedure when ascites is easily demonstrable on physical examination. When small amounts of ascites are present, a fluid wave may be difficult to demonstrate even when 1.5 L ascites are present. CT scan or abdominal ultrasound guided needle aspiration is particularly useful in these cases. Patients with ascites from cirrhosis may develop SBP and yet have minimal evidence of infection; some patients may be completely asymptomatic.¹² A low threshold for performing paracentesis is recommended in this setting, despite the low-grade coagulopathy that frequently is seen.

Footnotes