What is the diagnosis for a 63-year-old male presenting with bilateral abdominal distension, normal computed tomography (CT) scan, leukocytosis (White Blood Cell (WBC) count 13,000) with mild neutrophilia and monocytosis, without fever, normal C-reactive protein (CRP), and normal lactate levels?

Ascites: Given the bilateral abdomen bulges, ascites is a strong consideration. The normal CT scan does not rule out ascites, as small amounts of fluid may not be visible. The slightly elevated WBC count with neutrophils and monocytes could indicate a mild inflammatory response, but the absence of fever and normal CRP and lactate levels suggest a non-infectious cause. Conditions like cirrhosis or heart failure could lead to ascites without significant infection signs.

Intra-abdominal fluid accumulation due to heart failure: This could explain the bilateral bulges and slightly elevated WBC count. Heart failure can lead to fluid buildup in the abdomen (ascites) and peripheral edema.
Nephrotic syndrome: Although less common, nephrotic syndrome can cause significant fluid retention leading to ascites and peripheral edema. The lack of fever and normal inflammatory markers does not rule out this condition.
Malnutrition or hypoalbuminemia: Low albumin levels can lead to fluid shifting into the abdominal cavity, causing ascites. This could be due to various reasons including liver disease, malnutrition, or nephrotic syndrome.

Intra-abdominal malignancy with ascites: Although the CT scan is normal, it's crucial not to miss a potential malignancy, especially in a 63-year-old. Some tumors can produce ascites without significant mass effect visible on CT.
Spontaneous bacterial peritonitis (SBP): Despite the normal CRP, lactate, and absence of fever, SBP is a critical diagnosis to consider in anyone with ascites, as it can present subtly and has a high mortality rate if untreated.
Tuberculous peritonitis: This condition can cause ascites and has a varied presentation, including mild inflammatory responses. It's less common but critical to diagnose due to its treatability and potential for severe outcomes if missed.

Chylous ascites: This is a rare condition where lymphatic fluid accumulates in the abdomen, often due to obstruction or disruption of lymphatic vessels. It could present with abdominal bulges and might not have significant inflammatory markers.
Pseudomyxoma peritonei: A rare condition characterized by recurrent mucinous ascites, often associated with a primary mucinous tumor. It's an important consideration in the differential for ascites, especially if other causes are ruled out.