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Difficulty: Very hardInterpreting Laboratory Results

A 5858-year-old man with a history of decompensated alcoholic cirrhosis presents to the emergency department with diffuse abdominal pain and a low-grade fever over the past 2424 hours. Vital signs are temperature 37.9∘C37.9^\circ C (100.2∘F100.2^\circ F), blood pressure 105/65105/65 mmHg, heart rate 9898/min, and respiratory rate 1818/min. Physical examination reveals scleral icterus and a distended, diffusely tender abdomen with a positive fluid wave. To evaluate for spontaneous bacterial peritonitis, a diagnostic paracentesis is performed. The procedure is complicated by a traumatic tap, yielding noticeably bloody fluid. The ascitic fluid laboratory analysis reveals the following:

Ascitic fluid red blood cell (RBC) count: 62,500/μL62,500/\mu L
Ascitic fluid white blood cell (WBC) count: 1,250/μL1,250/\mu L
Ascitic fluid differential: 50%50\% neutrophils, 30%30\% lymphocytes, 20%20\% macrophages

Based on these results, what is the corrected ascitic fluid absolute neutrophil count (in cells/μL\mu L) that should be used to evaluate the patient for spontaneous bacterial peritonitis?

Answer: 375 cells/\mu L

Answer

The corrected absolute neutrophil count is 375375 cells/μL\mu L.
In the evaluation of spontaneous bacterial peritonitis (SBP), the diagnostic threshold is an absolute neutrophil count (ANC) of ≥250\ge 250 cells/μL\mu L in the ascitic fluid. When a paracentesis is traumatic (bloody), peripheral blood contaminates the fluid, falsely elevating the WBC and neutrophil counts. To correct for this, clinicians must subtract 11 neutrophil for every 250250 red blood cells (RBCs) in the fluid. First, calculate the uncorrected ANC: 1,250 WBCs×0.50=6251,250 \text{ WBCs} \times 0.50 = 625 neutrophils/μL\mu L. Next, calculate the correction factor: 62,500 RBCs/250=25062,500 \text{ RBCs} / 250 = 250 neutrophils/μL\mu L. Finally, subtract the correction from the uncorrected ANC: 625−250=375625 - 250 = 375 cells/μL\mu L. Because the corrected value (375375) is ≥250\ge 250 cells/μL\mu L, the patient has SBP and requires empiric antibiotic therapy.

Step-by-Step Solution

1
Calculate the uncorrected absolute neutrophil count.
1,250×0.50=6251,250 \times 0.50 = 625 cells/μL\mu L
To establish the baseline number of neutrophils in the sample before accounting for peripheral blood contamination.
2
Calculate the expected neutrophil contamination from the traumatic tap.
62,500/250=25062,500 / 250 = 250 neutrophils/μL\mu L
Peripheral blood artificially elevates the ascitic neutrophil count. The widely accepted correction factor is subtracting 11 neutrophil for every 250250 RBCs present in the fluid.
3
Determine the corrected absolute neutrophil count.
625−250=375625 - 250 = 375 cells/μL\mu L
Subtracting the peripheral contamination from the uncorrected count isolates the true ascitic neutrophil response, determining if the patient meets the diagnostic threshold for spontaneous bacterial peritonitis (≥250\ge 250 cells/μL\mu L).

Key Concept

Correcting ascitic fluid absolute neutrophil count in the setting of a traumatic paracentesis
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Topics

Interpreting Laboratory Results

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