What calculations are done with peritoneal fluids?

From the Guidelines

Peritoneal fluid calculations primarily involve assessing the fluid's characteristics to diagnose conditions like peritonitis, ascites, or peritoneal dialysis adequacy, with the serum-ascites albumin gradient (SAAG) being a key calculation to determine portal hypertension. The most common calculations include the SAAG, which is determined by subtracting the ascitic fluid albumin from the serum albumin (values ≥1.1 g/dL suggest portal hypertension, while <1.1 g/dL indicates other causes) 1. For peritoneal dialysis patients, you should calculate the dialysate-to-plasma ratio of creatinine (D/P Cr) to determine membrane transport characteristics, and Kt/V urea and creatinine clearance to assess dialysis adequacy. The protein concentration ratio between peritoneal fluid and serum helps distinguish exudates (>0.5) from transudates (<0.5) 1. Cell counts are also important, with neutrophil counts >250 cells/mm³ suggesting bacterial peritonitis 1. When analyzing peritoneal fluid, always consider the clinical context and compare results with established reference ranges, as these calculations guide appropriate treatment decisions for conditions affecting the peritoneal cavity 1. Some key points to consider when analyzing peritoneal fluid include:

• The SAAG is a useful tool for determining the cause of ascites, with a value ≥1.1 g/dL suggesting portal hypertension 1
• Cell counts, including neutrophil counts, are important for diagnosing bacterial peritonitis 1
• The protein concentration ratio between peritoneal fluid and serum can help distinguish exudates from transudates 1
• Peritoneal dialysis patients require calculation of D/P Cr, Kt/V urea, and creatinine clearance to assess dialysis adequacy
• Clinical context and comparison with established reference ranges are crucial when analyzing peritoneal fluid 1

From the Research

Calculations for Peritoneal Fluids

To diagnose and manage ascites, several calculations can be performed on peritoneal fluids, including:

• Serum-ascites albumin (SAA) gradient: calculated as the serum albumin concentration minus the ascitic fluid albumin concentration 2, 3, 4, 5, 6
• Ascitic fluid total protein: used to classify ascitic fluids as exudates or transudates, although this method has been challenged 3, 6
• Ascites/serum total protein ratio: used as a marker for the differential diagnosis of ascites, although its diagnostic accuracy is lower than the SAA gradient 3
• Ascitic fluid lactic dehydrogenase concentration: used as a marker for the differential diagnosis of ascites, although its diagnostic accuracy is lower than the SAA gradient 3
• Ascites/serum lactic dehydrogenase ratio: used as a marker for the differential diagnosis of ascites, although its diagnostic accuracy is lower than the SAA gradient 3
• Ascitic fluid white blood cell and polymorphonuclear leukocyte (PMN) count: used to diagnose spontaneous bacterial peritonitis 5
• Ascitic fluid pH and blood-ascitic fluid pH gradient: used to diagnose spontaneous bacterial peritonitis 5

Interpretation of Results

The results of these calculations can be interpreted as follows:

• An elevated SAA gradient (≥ 1.1 g/dL) indicates portal hypertension, while a low gradient (< 1.1 g/dL) indicates no portal hypertension 2, 3, 5
• A high ascitic fluid PMN count (> 250 cells/μL) or a low ascitic fluid pH (< 7.35) indicates spontaneous bacterial peritonitis 5
• A high ascitic fluid total protein concentration can indicate an exudate, while a low concentration can indicate a transudate, although this method is not reliable 3, 6

Diagnostic Approach

The diagnostic approach to ascites should involve the following steps:

• Calculate the SAA gradient to differentiate ascites caused by portal hypertension from other causes 2, 3, 4, 5, 6
• Perform a cell count and bacterial culture of the ascitic fluid to diagnose spontaneous bacterial peritonitis 5
• Restrict additional biochemical testing to specific diagnostic queries and collaborate closely with the clinician 6