How is a refractometer used to assess plasma protein concentration?

• A. By measuring refractive index to estimate total solids, which correlate with plasma protein
• B. By measuring pH of plasma
• C. By counting cells in plasma
• D. By measuring viscosity

Answer: (A)

A refractometer works by measuring how much light bends as it passes through plasma. The degree of bending depends on how concentrated the dissolved substances are, and in plasma the main dissolved solids are proteins. As the protein concentration rises, the refractive index of the plasma increases, so the instrument’s reading on the appropriate scale increases correspondingly. You place a small drop of plasma on the refractometer prism and read the value, which is then used to estimate the plasma protein concentration (often expressed in g/dL) using a calibration or conversion to a protein reference scale. This method is a quick, practical screening for protein status, but it gives an estimate and can be affected by lipids or bilirubin. The other properties listed aren’t measured by a refractometer—pH requires a pH meter, cell counts require cellular counters, and viscosity requires a viscometer.