Spontaneous bacterial peritonitis laboratory findings

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] ; Associate Editor(s)-in-Chief: Aditya Govindavarjhulla, M.B.B.S. [2] Guillermo Rodriguez Nava, M.D. [3] Shivani Chaparala M.B.B.S [4]

Overview

• Diagnosis requires paracentesis (needle drainage of the ascitic fluid).
• Ascitic fluid culture is negative in up to 60% of patients with clinical manifestations of spontaneous bacterial peritonitis (SBP)
• Therefore, the diagnosis of SBP is based on the neutrophil count, which reaches its highest sensitivity with a cutoff neutrophil count of > 250/mm³.
• Leukocytosis and acidosis may be present, but they are non-specific findings.
• If reasonable doubt still persists, an exploratory peritoneal lavage may be performed (e.g. in the case of trauma, in order to look for white blood cells, red blood cells, or bacteria).

Laboratory Findings

Laboratory tests for the diagnosis and differential diagnosis of SBP
Ascitic fluid analysis	Normal	Spontaneous Bacterial Peritonitis	Secondary bacterial peritonitis	Hepatic ascites	Cardiac ascites	Nephrogenic ascites	Pancreatic ascites	Tuberculous ascites	Malignant ascites
Gross appearance		cloudy or turbid	turbid or purulent	clear straw or milky	clear to pale yellow	straw colored or chylous	milky or cloudy or turbid	milky or normal	milky or bloody
Leukocyte count and differential		≥ 250 PMN	> 1000 predominantly PMN	≥ 250 PMN or normal	-	< 250 mononuclear	-	≥ 250 PMN or normal	-
Total protein		≥ 25 g/L	> 25 g/ L	< 25 g/L	< 25 g/L	< 25 g/L	≥ 25 g/L	≥ 25 g/L	≥ 25 g/L
Serum-ascites albumin gradient		< 1.1 g/dL		≥ 1.1 g/dL	≥ 1.1 g/dL		< 1.1 g/dL	< 1.1 g/dL	< 1.1 g/dL
LDH(lactate dehydrogenase)		↑or normal	> upper limit of normal for serum LDH	↓	↓ or normal		↑or normal	↑or normal	↑
Glucose		↓	< 50 mg/dL	normal	normal		↓	↓	↓
Amylase		-		normal	-		↑	-	↑or normal
Tumor markers		-		↑or normal	normal		↑or normal	↑or normal	↑
Additional analyses		bacteriology	bacteriology, CEA >5 ng/ml Alkaline phosphatase > 240 units/ L					laparoscopy, peritoneal biopsy, bacteriology, PCR.	cytology

Approach to the diagnosis and treatment of spontaneous bacterial peritonitis

Diagnostic Paracentesis ❑ Perform ascitic fluid cell count and differential ❑ Perform ascitic fluid culture (Inoculated at bedside)

PMN ≥ 250cells/mm³

If YES ❑ Presumptive SBP ❑ Begin empiric antibiotic therapy(eg:Cefotaxime 2g IV q8H and ❑ IV Albumin on day 1 & day 3 IF serum creatinine 1mg/dl, BUN > 30mg/dl or total albumin > 4mg/dl

IF NO ❑ Look for the signs/symptoms of Infection

Is Culture Positive ?

Absent Symptoms ❑ Is Culture Positive?

Symptoms Present ❑ Begin Empiric Antibiotic Therapy for SBP

Negative Culture ❑ Complete 5 day Antibiotic Course

Confirmed SBP ❑ Narrow the spectrum based on the susceptibility to complete the 5 day course

Culture Negative ❑ No Antibiotics indicated

Culture Positive ❑ Bacterascites: Repeat diagnostic paracentesis when the culture growth is discovered

Routine laboratory studies for spontaneous bacterial peritonitis include:

• Complete blood count and differential count to confirm infection.
• BUN, S.creatinine to asses the renal function.
• ABG analysis
• S.glucose
• Blood culture may be useful in sepsis
• Serum electrolytes
• Liver Function tests and serum.bilirubin to assess the liver function.
• Coagulation profile
• Urine analysis and culture to rule out asymptomatic bacteriuria
• Amylase and Lipase levels to rule out pancreatitis as the cause of ascites

Diagnostic paracentesis:

Performed usually within 72hrs of admission.

Indications for diagnostic paracentesis
All patients with cirrhosis and ascites and with any of the following:
Upon admission to the hospital and/or,
With a change in the clinical course including development of fever, abdominal pain, altered mental status, ileus, or septic shock.
Development of laboratory abnormalities such as a leukocytosis, acidosis, or renal failure.
During episodes of gastrointestinal bleeding prior to the administration of antibiotics.
Worsening of liver function and Hepatic encephalopathy.

Given the similarities in presentation between the variants of ascitic fluid infection and the inability to clinically distinguish spontaneous from secondary peritonitis, the diagnosis of SBP should be through abdominal paracentesis as it has been shown to be safe with a low risk for complication, even in patients with marked coagulopathy and thrombocytopenia.

Ascitic Fluid Analysis

• Ascitic fluid analysis is the gold standard and is required for the confirmation of the diagnosis of spontaneous bacterial peritonitis.
• As high index of suspicion must exist for ascitic fluid infection in a cirrhotic patient.
• The following tests are recommended for suspected infection of the ascitic fluid:

1. Cell count with differential : Absoulte neutrophil count - a total count of >250 cells/mm³ confirm the diagnosis of spontaneous bacterial peritonitis.

• It is the most sensitive and single best test in diagnosing ascitic fluid infection.
• Although, not all cases in which the PMN count is above this threshold represent infection, values in this range can also be seen with hemorrhage into ascites, peritoneal carcinomatosis, or pancreatic ascites.
• A useful distinguishing feature is that the PMN is usually not the predominant cell type in these cases

1. Ascitic fluid culture.

• Performed before initiating antibiotic therapy by
• By bedside inoculation of ascitIc fluid ≥ 10 mL into blood culture bottles, instead of sending the fluid to the laboratory in a syringe or container, since immediate inoculation improves the yield on bacterial culture from approximately 65% to 90%, when the ascitic fluid cell count is at least 250 cells/mm3.
• Separate and simultaneous blood cultures should also be obtained, as up to 50% of patients with SBP have concomitant bacteremia.
• When culture is positive, the most common organisms are Gram-negative bacteria (mainly Escherichia coli) and Gram-positive cocci (usually Streptococcus spp. and enterococci).
• Neither sensitive/specific.
• But is indicated to rule out secondary peritonitis caused by many organisms usually anaerobes and also to guide and narrow down the appropriate antibiotic of choice based on the culture and sensitivity results.

1. Gram stain to identify the number of organisms causing peritonitis.
2. Total protein
3. Lactate dehydrogenase
4. Glucose
5. Amylase
6. Albumin (if SAAG unknown) concentration - it is important for the calculation of serum-ascites albumin gradient , and helps us in identifying the portal hypertension and associated prognosis.
7. Serum-ascites albumin gradient (if not calculated before)
8. AFB smear and culture

• Paracentesis reveals an ascitic fluid with, most commonly,
  • A total white cell count of up to 500 cells/mcL with a high polymorphonuclear (PMN) cell count (250/mm³ more) and a
  • Protein concentration of 1 g/dL (10 g/L) or less,low protein concentration is noticed in cases of spontaneous bacterial peritonitis which differentiates it from secondary bacterial peritonitis where it is normal.
  • Corresponding to decreased ascitic opsonic activity.
  • Some patients may have an ascitic neutrophil count <250 cells/mm³ with positive cultures. This is known as "bacterascites".
  • These patients should undergo a repeat paracentesis.
  • Patients with signs of Systemic inflammatory response syndrome (SIRS) or in whom the repeat ascitic neutrophil count is >250 cells/mm³ should receive antibiotic therapy.
  • If not, they should be followed up.

• Absoulte neutrophil count - a total count of >250 cells/mm³ confirms the diagnosis of spontaneous bacterial peritonitis.

• • If the diagnosis is doubtful, the serum procalcitonin level has a 95% sensitivity, and 98% specificity with a 0.75 ng/mL cutoff.

• Recently leukocyte esterase calibrated reagent strips (LERS) to assess the PMN cell count (cut-off of > 250 PMN/mcL) are promised to provide good screening results when the strip turns any hue of tan/brown at 3 min.
• High degree of sensitivity, but the sensitivity is too low for routine use.
• Given the rapidity (90 s), low cost, and availability ‘dipstick’ testing of ascites allows for more rapid diagnosis and management of SBP.

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