- Design
- Retrospective single-centre observational study, ROC analysis
- Population
- 261 patients with malignant ascites, 337 paracenteses (19 culture-positive)
- Primary outcome
- Fluid characteristics associated with positive ascitic culture
- Effect
- PMN ≥250: sensitivity 73.7%, specificity 72.4%; PMN >1,318: 68.4% and 93.0%
A retrospective single-centre study in the Journal of Hospital Medicine (18 September) reviewed 337 paracenteses in 261 patients with malignant ascites and a full fluid work-up. Nineteen samples, from 17 patients, grew organisms.
The cirrhosis threshold for spontaneous bacterial peritonitis — 250 PMN cells/mm3 — had a sensitivity of 73.7% and specificity of 72.4% here. So roughly one in four culture-negative taps crossed it. The best thresholds found were a PMN percentage of 63% or more (sensitivity 68.4%, specificity 90.8%) and a PMN count above 1,318 cells/mm3 (sensitivity 68.4%, specificity 93.0%).
Neither new threshold is ready for use: they were derived from only 19 positive cultures and have not been validated. What the study does support is caution. In malignant ascites a PMN count just over 250 is not the diagnosis it would be in cirrhosis, and a count below it does not exclude infection.
- Always send cultures in blood culture bottles at the bedside, not only a cell count
- Record the PMN percentage as well as the absolute count
- Weigh the clinical picture — fever, pain, rising inflammatory markers — above the 250 cut-off
- Review empirical antibiotics once cultures return rather than completing a full SBP course by default
Why it matters
It challenges the reflex of applying a cirrhosis rule to every ascitic tap on the ward.
Don't overread it
The proposed cut-offs come from 19 positive cultures at one centre and are not validated for clinical use.
The statistics, in plain English
Specificity of 72% means about 28 of every 100 uninfected samples would be called infected at 250 cells/mm3. The better-looking thresholds came from the same small dataset they were tested on, which flatters them; with only 19 positives, the sensitivity estimates carry wide uncertainty.
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