The gold standard test for SBP is ascitic fluid analysis with measurement of the PMN. It is useful for the diagnosis and monitoring of treatment. The culture of the ascitic fluid may be positive if was done correctly .
There is a variant of SBP that is called culture-negative neutrocytic ascites. It is characterized by elevated ascitic fluid PMN but the culture is negative. It is managed exactly as classic SBP. Such cases would be missed if cultures were not done .
The manual PMN counting is time consuming, laborious and required some experience to avoid intra- and inter-observer variability. So, a simple rapid bedside test would be useful clinically .
Calprotectin is acute-phase inflammatory protein that is released from the PMN. Calprotectin has anti-proliferative and antimicrobial properties . Calprotectin is used clinically widespread in the diagnosis and monitoring treatment of inflammatory bowel disease .
In an earlier study , patients with liver cirrhosis had higher fecal calprotectin compared with the control. Fecal calprotectin correlated with hepatic encephalopathy grade and SBP.
Later on, studies were conducted on ascitic calprotectin [12, 14, 16,17,18]. Ascitic calprotectin could be measured by either enzyme-linked immunosorbent assay (ELISA) or a point-of-care (POC) lateral flow assay with the Quantum Blue® Reader .
Burri et al.  reported that patients with SBP had statistically higher values of ascitic calprotectin that was measured by two techniques namely ELISA and POC. A cutoff value (0.63 μg/mL) measured by ELISA had 94.8% sensitivity, 89.2% specificity, 60% PPV and 99% NPV. A cutoff value (0.51 μg/mL) measured by PCO had 100% sensitivity, 84.7% specificity, 100% PPV and 87.7% NPV. Both techniques were useful with excellent correlation.
Two studies assessed the ratio of ascitic calprotectin to ascitic total protein. One study  found that ratio was useful for SBP diagnosis unlike the other one .
Fernandes et al.  studied 88 patients of whom 41 had SBP. They were mainly males and alcoholics. Higher ascitic calprotectin was found. A cutoff value (1.57 μg/mL) measured by POC had 87.8% sensitivity, 97.9% specificity, 97.3% PPV and 90.2% NPV.
In the study conducted by Abdel-Razik et al. , patients with SBP had higher calprotectin, serum procalcitonin, serum and ascites TNF-α, IL-6. A cutoff value of 94 ng/mL had 94.3% sensitivity, 91.8% specificity, 93% PPV and 95% NPV.
In a recent French study , a 1.51 μg/mL cutoff measured by POC had 86.1% sensitivity, 92% specificity, 65.9% PPV and 97.3% NPV.
CRP is acute-phase protein that elevates in many inflammatory conditions. The hsCRP measures the low levels of CRP. The hsCRP has a higher sensitivity than CRP. Patients with SBP show elevated CRP levels that decrease with treatment [12, 14, 19]. Only one study by Guler et al.  assessed the role of serum hsCRP in non-neutrocytic ascites. They found that hsCRP was higher in patients with SBP and non-neutrocytic ascites compared with the control. The levels decreased with antibiotic therapy. Leukocyte esterase is an enzyme produced by PMN in response to inflammation. It can be detected by leucocyte esterase reagent strips.
In the current study, the measurement of serum or ascitic nitrous oxide did not add a benefit in SBP diagnosis. Patients with SBP had higher serum CRP level, which is consistent with the other studies [12, 14, 19]. Furthermore, serum hsCRP was higher in SBP patients in accord with Guler et al.  study. Most patients had positive ascitic leucocyte esterase test (95%) in contrast few patients in the control group.
Calprotectin was significantly higher in SBP patients. Calprotectin >2 ng/mL had 90% sensitivity, 92.5% specificity, 92.3% positive predictive value and 90.2% negative predictive value. The cutoff is completely different from other studies [11, 14, 16, 18] even measured by ELISA but the sensitivity and specificity are high as reported by other studies [11, 14, 16, 18]. In fact, the methodology of measurement should be standardized in the next studies.
Limitations of the study
The number of patients is small, we did not follow the treatment measuring calprotectin and so the recurrence.