A 56-year-old man with a history of alcohol use disorder and compensated cirrhosis present… — USMLE Step 3

A $56$ -year-old man with a history of alcohol use disorder and compensated cirrhosis presents to the emergency department with severe, diffuse abdominal pain, nausea, and vomiting for the past $24$ hours. He has not had any recent gastrointestinal bleeding. His temperature is $38.6^\circ\text{C}$ ( $101.5^\circ\text{F}$ ), blood pressure is $90/60\text{ mmHg}$ , heart rate is $112/\text{min}$ , and respiratory rate is $22/\text{min}$ . Physical examination reveals a distended abdomen with diffuse tenderness, guarding, and rebound tenderness. Bowel sounds are absent.

Laboratory studies are obtained, and a diagnostic paracentesis is performed.

Laboratory Test	Result	Reference Range
Serum
Albumin	$2.8\text{ g/dL}$	$3.5-5.0\text{ g/dL}$
Total protein	$6.2\text{ g/dL}$	$6.0-7.8\text{ g/dL}$
Lactate dehydrogenase (LDH)	$180\text{ U/L}$	$100-250\text{ U/L}$
Amylase	$65\text{ U/L}$	$25-125\text{ U/L}$
Ascitic Fluid
Color	Turbid	Clear to pale yellow
Leukocyte count	$5,500/\text{mm}^3$	$< 500/\text{mm}^3$
Neutrophils	$85\%$	$< 50\%$
Albumin	$1.2\text{ g/dL}$	--
Total protein	$3.4\text{ g/dL}$	--
Glucose	$28\text{ mg/dL}$	Similar to serum
Lactate dehydrogenase (LDH)	$640\text{ U/L}$	$< 50\%$ of serum limit
Amylase	$45\text{ U/L}$	$< 50\text{ U/L}$

Based on the provided laboratory results, which of the following is the most likely diagnosis?

A
Spontaneous bacterial peritonitis
B
Tuberculous peritonitis
Secondary bacterial peritonitisAnswer
D
Pancreatic ascites
E
Malignant ascites

Answer

Secondary bacterial peritonitis

The correct answer is Secondary bacterial peritonitis. The patient has a serum-ascites albumin gradient (SAAG) of

1.6\text{ g/dL}

, which confirms portal hypertension. The absolute neutrophil count (ANC) of

4,675/\text{mm}^3

(calculated as

85\%

5,500/\text{mm}^3

) is well above the

\geq 250/\text{mm}^3

threshold for diagnosing peritonitis. Crucially, the differentiation between spontaneous bacterial peritonitis (SBP) and secondary bacterial peritonitis (caused by a perforated abdominal viscus) relies on Runyon's criteria. Secondary peritonitis is diagnosed when at least two of the following three criteria are met in the ascitic fluid: total protein >

1.0\text{ g/dL}

, glucose <

50\text{ mg/dL}

, and LDH greater than the upper limit of normal for serum. This patient meets all three criteria. Recognizing this pattern is critical because secondary peritonitis requires urgent imaging (e.g., upright abdominal X-ray or CT) and surgical exploration, whereas SBP is managed purely medically.

Step-by-Step Solution

Calculate the serum-ascites albumin gradient (SAAG).

SAAG = Serum Albumin (

2.8\text{ g/dL}

) - Ascitic Albumin (

1.2\text{ g/dL}

) =

1.6\text{ g/dL}

A SAAG

\geq 1.1\text{ g/dL}

confirms the presence of portal hypertension, which is expected given the patient's history of cirrhosis.

Calculate the absolute neutrophil count (ANC) in the ascitic fluid.

ANC =

5,500/\text{mm}^3 \times 0.85 = 4,675/\text{mm}^3

An ANC

\geq 250/\text{mm}^3

diagnoses peritonitis and necessitates prompt treatment.

Evaluate the ascitic fluid chemistry against Runyon's criteria to distinguish spontaneous from secondary peritonitis.

The ascitic fluid has total protein >

1.0\text{ g/dL}

(

3.4\text{ g/dL}

), glucose <

50\text{ mg/dL}

(

28\text{ mg/dL}

), and LDH > upper limit of normal for serum (

640\text{ U/L}

250\text{ U/L}

Meeting at least two (here, all three) of Runyon's criteria strongly suggests secondary bacterial peritonitis (e.g., from a perforated viscus) rather than spontaneous bacterial peritonitis.

Key Concept

Interpreting ascitic fluid analysis to differentiate spontaneous from secondary bacterial peritonitis using Runyon's criteria.

Question

Answer

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Key Concept

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