Clinical Information

Specific gravity (SG), the ration of the mass of a solution compared to the mass of an equal volume of water, is an estimate of the concentration of substances dissolved in the solution.

Accessing whether a body fluid specimen is exudative or transudative in nature is the initial step in determining the etiology of the fluid. Transudative fluids result from hemodynamic aberrations or oncotic changes and are associated with ultrafiltration of serum across membranes. Transudates most commonly occur in association with clinically apparent conditions such as heart failure and cirrhosis. Exudative fluids tend to develop as a consequence of inflammation or malignant disorders such as tuberculosis, pneumonia or cancer, in which capillary permeability is increased, allowing large-molecular-weight compounds to be released into the accumulating fluid. If the fluid is transudate, further diagnostic procedures are often not necessary, however the presence of an exudative fluid often triggers additional testing that may be invasive in nature.

Determination of body fluid SG can aid in the distinction between transudative and an exudative fluid. SG in exudates is greater than in transudates. This same information can be obtained from the total protein using 3 g/dL as the cutoff.