Commentary on A Patient with Third-Space Fluid Loss.

Abstract

The “third space” describes body compartments that sequester fluid that does not freely equilibrate with the plasma. Tay et al. describe a patient with third-space fluid accumulation in the pleural and peritoneal spaces. Interpretation of biochemical parameters measured in both fluids was important in determining the underlying diagnosis. However, it is equally important to recognize the limitations of the criteria used to assess pleural and peritoneal effusions so that the correct conclusions can be drawn in complex cases such as this. Light criteria have stood the test of time as a vital component of pleural fluid analysis, capably separating effusions into transudates and exudates. In this case, Light criteria classify the effusion as a transudate. A limitation of Light criteria is the false classification of approximately 20%–30% of transudates as exudates. However, the criteria have a high negative predictive value for the identification of exudates, and so it is highly likely that this patient’s effusion is indeed a transudate.