Comparison of biuret and refractometry methods for the serum total proteins measurement in ruminants

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Determination of serum total protein concentration is commonly performed by the biuret method. Refractometric measurement is a faster and less expensive alternative but its accuracy has not been determined in ruminants.


The purpose of the study was to compare the serum total protein concentrations in cattle, sheep, and goats measured by the biuret method with those obtained by refractometry.


Serum total protein concentration was determined in 120 cattle, 67 sheep, and 58 goat blood samples refractometrically and with the biuret method. The data were analyzed with a paired samples t-test, and Passing and Bablok regression equations and Bland and Altman plots were generated.


There was a strong linear relationship between the total protein values determined with the refractometer and the biuret method in cattle, sheep, and goats. The statistical accuracy, which represents a bias correction factor that measures the deviation of the best-fit line from the 45° line through the origin, was 90.63% for cattle, 93.05% for sheep, and 91.76% for goats. The mean protein values determined with the refractometer were significantly lower than those measured with the biuret method in cattle and goats (P < .05) but not in sheep (P > .05).


The evaluated refractometer was sufficiently accurate for the determination of serum total proteins in cattle, sheep, and goats, although it cannot be used interchangeably with the biuret method. The RIs should be corrected for negative bias based on the created equations.

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