TOTAL DISSOLVED SOLIDS IN PLURAL FLUID: A PILOT STUDY COMPARING TDS ANALYSIS TO LIGHTS CRITERIA FOR EXUDATE / TRANSUDATE DIFFERENTIATION

Abstract

Background: Differentiating exudative from transudative pleural effusions is critical for identifying underlying pathology and informing management. While Light’s criteria remains the gold standard, its reliance on laboratory infrastructure and time-consuming measurements can limit its application in resource-constrained settings. This pilot study assessed the diagnostic utility of measuring Total Dissolved Solids (TDS) in pleural fluid as a rapid, bedside adjunct for distinguishing exudates from transudates and compared its performance to Light’s criteria.

Methods: A pilot study was conducted in the Department of Pulmonology at Saveetha Medical College and Hospital over a three-month period, from March to May 2025. Thirty-five adult patients with radiologically confirmed pleural effusions underwent diagnostic thoracentesis. Pleural fluid TDS was measured using a calibrated handheld meter and compared with conventional biochemical parameters. Effusions were classified as exudative or transudative based on Light’s criteria. Statistical analyses assessed the correlation of TDS with established markers using t-test.

Results: Among the 35 cases, 22 (62.9%) were exudative and 13 (37.1%) were transudative. Mean TDS values were significantly higher in exudates (490±110mg/L) than in transudates (310±90mg/L). TDS levels showed strong concordance with biochemical markers, particularly pleural fluid protein and LDH. Statistical analysis revealed significant associations between TDS and markers of exudation (p<0.05), supporting its discriminative potential.

Conclusion: TDS measurement in pleural fluid offers a simple, rapid, and cost-effective adjunct to traditional analysis for distinguishing exudative from transudative effusions. While closely aligned with Light’s criteria, its ease of use and independence from laboratory resources make it particularly valuable in emergency and low-resource settings. Further studies with larger cohorts are warranted to validate diagnostic thresholds and facilitate wider clinical adoption.