Final answer:
SAAG is necessary to diagnose spontaneous bacterial peritonitis and is calculated by subtracting the ascitic fluid albumin level from the serum albumin level.
Step-by-step explanation:
To diagnose spontaneous bacterial peritonitis (SBP), a diagnostic test called SAAG (Serum-ascites albumin gradient) is used. SAAG is calculated by subtracting the ascitic fluid albumin level from the serum albumin level. SBP is diagnosed if the SAAG is less than 1.1 g/dL. SAAG is necessary because it helps differentiate SBP from other causes of ascites and also determines the severity of the condition.
For example, if a patient has ascites, an elevated white blood cell count in the ascitic fluid, and a SAAG less than 1.1 g/dL, it suggests the presence of SBP. On the other hand, if the SAAG is greater than 1.1 g/dL, it indicates a different cause of ascites such as liver cirrhosis.
Therefore, SAAG is an important diagnostic tool to accurately identify and diagnose spontaneous bacterial peritonitis.