Assay of cerebrospinal fluid protein: a rate biuret method evaluated

We evaluated a rate colorimetric method (Beckman) for measuring total protein in cerebrospinal fluid. The automated instrument we used was Beckman's ASTRA TM. A 100-microL sample of spinal fluid is introduced into the biuret reagent in the reaction cell and the increase in absorbance at 545 nm is monitored for 20.5 s. Solid-state circuits determine the rate of alkaline biuret-protein chelate formation, which is directly proportional to the total protein concentration in the sample. The linear range of measurement is 120 to 7500 mg/L. Day-to-day precision (CV) over the range of 150 to 1200 mg/L ranged from 15.2 to 2.3%. The method was unaffected by radical alteration of the albumin/globulin ratio, but there is a positive interference in the presence of hemoglobin, a suppression in the presence of bilirubin, and no effect by xanthochromia. The method is precise, accurate, rapid, and convenient. The method was compared with the trichloroacetic acid method as performed on the Du Pont aca III, giving a correlation coefficient (r2) of 0.9693. The method is precise, accurate, rapid, and convenient.