#996 Validation of peritoneal NGAL as a biomarker for peritonitis: a comparison between laboratory-base method and rapid stick test
Authors: Virzi Grazia Maria , Morisi Niccolò , Manani Sabrina Milan , de Cal Massimo , Tantillo Ilaria , Donati Gabriele , Ronco Claudio , Zanella Monica 
Published: May 2024

Abstract

Background and Aims
Neutrophil gelatinase-associated lipocalin (NGAL) is a protein involved in iron homeostasis, inflammation, and fibrosis. Initially discovered in human granulocytes, NGAL is found in various tissues. In renal contexts, NGAL's small size (23-25 kDa) and positive charge aid glomerular filtration and absorption in the proximal tubule. Notably, elevated NGAL levels were identified in peritoneal fluids during peritonitis in 2009, establishing it as a biomarker for early Peritoneal Dialysis (PD)-associated peritonitis diagnosis for values more than 100 μg/L. While our medical centre currently relies on the lab-based NGAL assay (NGALlab), advancements introduce the NGAL dipstick test (NGALds), offering a rapid and semi-quantitative colorimetric assessment. This study compares NGALds with NGALlab in clinical practice, assessing NGALds's potential for early peritonitis detection in PD patients' peritoneal fluid.

Method
This study involves a validation of diagnostic accuracy conducted through a retrospective analysis of parallel samples obtained from peritoneal effluents. Samples were collected from both healthy patients during routine analysis and from cases with suspected or confirmed peritonitis. Both NGALlab and NGALds analyses were conducted on each sample. The analysis aimed to evaluate the performance of the new NGALds by comparing it with the established NGALlab. Peritoneal NGALlab quantification employed the standard BioPorto test (BioPorto Diagnostics, Hellerup, Denmark), utilizing a particle-enhanced turbidimetric immunoassay. The novel NGALds is a rapid semi-quantitative assay that measures NGAL levels in biological fluids through colourimetric test strips, utilizing an antibody sandwich lateral flow dipstick test. NGAL values were categorized into colour-coded levels: 25 μg/L, 50 μg/L, 100 μg/L, 150 μg/L, 300 μg/L, and 600 μg/L. Statistical analyses were performed using SPSS Statistics 26.0 software. Spearman rank correlation coefficients (Rs) assessed categorical groups of NGAL values for peritoneal NGALds and NGALlab tests. A graphic comparison of these two tests was presented through a Bland-Altman plot

Results
Out of 301 peritoneal effluent samples, 156 were diagnosed with suspected peritonitis by ISPD Criteria. As Table 1 showed, NGALds showed a strong correlation with NGALlab categories (Rs = 0.876, P = 10−96), successfully diagnosing 96% (150 out of 156) peritonitis cases. Discrepancies between NGALlab and NGALds, visualized in the Bland-Altman plot (Fig. 1), were most notable when NGAL values exceeded the 600 μg/L threshold. However, this difference doesn't impact clinical significance, as high NGAL values remain diagnostically for peritonitis with both methods.

Conclusion
In conclusion, our study underscores a significant correlation between NGAL values measured by the laboratory method (NGALlab) and the NGAL dipstick test (NGALds). Importantly, NGALds exhibited a strong capability, correctly identifying 96% of peritonitis cases determined by NGALlab. Beyond statistical correlation, the NGAL dipstick emerges as a valid alternative for swift evaluation in patients presenting symptoms suggestive of peritonitis. Its ease of use, rapidity, and semi-quantitative colorimetric assessment make NGALds a patient-friendly tool. This user-friendly characteristic holds immense potential for streamlined clinical workflows, allowing for prompt and efficient peritonitis diagnosis.