Measure the albumin concentration in ascites and serum and calculate the serum-to-ascites albumin gradient (SAAG) (serum minus ascitic albumin concentration).

A SAAG of 11 g/L or greater indicates portal hypertension with 97% accuracy, while a SAAG of <11 g/L indicates the absence of portal hypertension.

Causes of ascites according to the SAAG are given in Table 24.2.

Send a sample in an EDTA tube to the haematology laboratory for total and differential white cell count.

In uncomplicated cirrhosis, the total white cell count is <500/mm³ and neutrophil count <250/mm³.

Spontaneous bacterial peritonitis is associated with a neutrophil count of >250/mm³.

In peritoneal tuberculosis, the white cell count is usually 150–4000/mm³, predominantly lymphocytes.

Send ascites for culture in patients with new-onset ascites or if you suspect infection (fever, abdominal pain, confusion, renal failure or acidosis)

Inoculate aerobic and anaerobic blood culture bottles with 10 mL per bottle of ascites.

Send a sample for cytology if you suspect malignancy or if the SAAG is <11g/L.

Cytology is usually positive in the presence of peritoneal metastases, but these are found in only about two-thirds of patients with ascites related to malignancy.

Total protein

Glucose

LDH

Gram stain

Ziehl-Neelsen stain and testing for Mycobacterium tuberculosis DNA if suspected tuberculosis

Amylase if suspected pancreatitis