Specific Gravity, Body Fluid
Clinical Information Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
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.
An aid in determining the type of body fluid: exudate versus transudate
Exudate fluid specific gravity (SG) is >1.015; transudate fluid SG is <1.015
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
Cannot be performed on viscous fluids
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
No established reference values
Clinical References Provides recommendations for further in-depth reading of a clinical nature
Romero-Candeira S, Hernandez L: The separation of transudates and exudates with particular reference to the protein gradient. Curr Opin Pulm Med 2004 Jul;10(4):294-298