The indicator-dilution method is used to measure body fluid compartments. Which indicator correctly measures total body water (TBW)?

• A Inulin — distributes into TBW freely
• B Evans blue (T-1824) — distributes into plasma volume
• C 51Cr-labeled RBCs — distributes into TBW
• D Deuterium oxide (D2O) or tritiated water (THO) — distributes into TBW ✓

Explanation

Deuterium oxide (D2O) and tritiated water (THO/3H2O) are isotopic forms of water that distribute freely throughout all body fluid compartments (intracellular, extracellular, and transcellular), making them ideal tracers for TBW. Inulin distributes in the extracellular fluid (and is used to measure GFR and ECF volume, not TBW). Evans blue dye binds to plasma albumin and remains in the intravascular space, making it a plasma volume indicator. 51Cr-labeled RBCs measure red cell mass/blood volume. ICF volume cannot be measured directly; it is calculated as TBW minus ECF volume.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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