Total Protein

Refractometry: This method is used for estimating plasma protein (including fibrinogen) in EDTA plasma and is reported on routine hemograms at Cornell University. It measures the refractive index of a sample relative to the refractive index of water. The reading is actually a measurement of total solids and is only an estimate of protein concentration, since variation in other serum components "solids" (sodium, chloride, phosphate, glucose, cholesterol, urea, etc.) also can affect refractive index. Lipemia and moderate to severe hemolysis renders the results invalid.

Biuret Method: This is the colorimetric method used on the automated chemistry analyzer. It detects all proteins and is accurate for the range of 1-10 g/dl. It is not sensitive enough for low concentrations found in some body fluids such as CSF, urine, and many body cavity effusions.
Note that the total protein (as measured by refractometer) provided with the hemogram is usually higher than that provided from the chemistry analyzer. This is due to:
  1. The contribution of total solids to the refractive index.
  2. The contribution of fibrinogen to total protein content in plasma compared to serum. Note that if the chemistry panel is performed on heparinized plasma, the difference between the two measurements is much less as fibrinogen is present in both EDTA and heparinized plasma samples.
Turbidometric methods: Quantitative of protein in CSF, urine and other low-protein fluids requires more sensitive techniques than either the Biuret or refractometer method. Protein in these fluids can be measured more accurately with precipitation or dye-binding methods. Precipitation methods include trichloroacetic acid and sulfosalicylic acid, whereas dye-binding uses dyes such as Coomassie blue and pyrogallol red-molybdate. The procedure on the Hitachi 911 at Cornell University for measurement of protein in low-protein fluids is a turbidometric method based on precipitation of protein by benzethonium chloride. It is sensitive to as little as 6 mg/dL of protein.
Note that urinary dipsticks can be used to estimate protein in CSF samples, however they are not as accurate as turbidometric techniques.

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