The colloid osmotic pressure (COP) osmometer is characterized by which feature?

• A. Utilizes a cooling bath set at -7°C
• B. Measures total serum osmolality
• C. Negative pressure on reference saline side equivalent to COP of sample
• D. Measures contribution of electrolytes to osmolality

Answer: (C)

Colloid osmotic pressure is the pressure generated by large plasma proteins, such as albumin, that tends to pull water toward the vascular compartment. A COP osmometer measures this specific pressure by using a semipermeable membrane between the plasma sample and a reference saline solution. Water would naturally move from the saline side into the protein-rich sample due to the oncotic effect, so the instrument applies a controlled negative pressure to the saline side to oppose that flow. When the net water movement is halted, the magnitude of the balancing negative pressure equals the colloid osmotic pressure of the sample. This is why the correct feature is the negative pressure on the reference saline side that matches the COP of the sample.

The other ideas don’t fit: a cooling bath at -7°C is not how COP is measured and is more associated with other types of osmometers; COP measurement does not assess total serum osmolality, which includes all solutes, nor does it quantify the electrolyte contribution to osmolality—the COP method isolates the effect of colloids (proteins) rather than electrolytes.