Is the acidity of ascitic fluid a reliable index in making the presumptive diagnosis of spontaneous bacterial peritonitis?

Ascitic fluid pH and arterial-ascitic fluid pH gradient were compared to ascitic fluid polymorphonuclear cell count in 84 patients with cirrhotic ascites and in 12 with malignant ascites to assess their role as diagnostic tests for spontaneous bacterial peritonitis and to clarify the relationship between ascitic fluid pH and lactate. Ascitic fluid pH was significantly lower (pH 7.30) in spontaneous bacterial peritonitis (n = 18) and probable spontaneous bacterial peritonitis (n = 12) than in sterile ascites (pH 7.41; n = 54). Since blood pH levels were not different in the presence of infection, arterial-ascitic fluid pH gradient was significantly higher in spontaneous bacterial peritonitis and probable spontaneous bacterial peritonitis than in sterile ascites (0.12 vs. 0.02). The close correlations between arterial-ascitic pH gradient and lactate (r = 0.77, p less than 0.0001), lactate and bicarbonate gradient (r = 0.64, p = 0.003) and arterial-ascitic pH gradient and pCO2 gradient (r = -0.90, p less than 0.0001) suggest that the low ascitic fluid pH may be due to an increase in lactate and CO2. Patients with Escherichia coli-induced spontaneous bacterial peritonitis had significantly lower ascitic fluid pH and higher lactate than those with spontaneous bacterial peritonitis by other organisms. Values of ascitic fluid pH, lactate and arterial-ascitic fluid pH gradient in malignant ascites were similar to those of spontaneous bacterial peritonitis and probable spontaneous bacterial peritonitis. Cutoff points, selected by receiver operating characteristic curves analysis, of 450 per mm3 for polymorphonuclear cells and of 0.07 for arterial-ascitic fluid pH gradient, allow high positive and negative predictive values for spontaneous bacterial peritonitis.(ABSTRACT TRUNCATED AT 250 WORDS)