We developed a candidate Reference Method for measuring total serum protein by use of the biuret reaction. The method involves a previously described biuret reagent (Clin. Chem. 21: 1159, 1975) and Standard Reference Material (SRM) 927 bovine albumin (National Bureau of Standards) as the standard. At 25 degrees C, color development for 30 or 60 min provides identical serum protein values. Glucose (up to 10 g/L) and bilirubin (up to 300 mg/L) do not interfere. Hemoglobin, at 3 g/L, increases apparent serum protein by 0.4 g/L. The presence of dextran in serum causes easily detected turbidity, but this interference can be eliminated by centrifuging the reaction mixture. Therapeutic concentrations of ampicillin, carbenicillin, penicillin, oxacillin, nafcillin, chloramphenicol, cephalothin, and methicillin in blood do not interfere, nor do triglycerides up to 10 g/L. Within-run and day-to-day standard deviations of the method are 0.1 and 0.4 g/L, respectively.
Analysis of a chronological trend in data from the second National Health and Nutrition Examination Survey indicated that average blood lead levels in the United States dropped approximately 37 per cent (5.4 micrograms per deciliter) from February 1976 through February 1980. There was no evidence that this trend was due to errors in laboratory measurement or to the design of the survey. The trend was present even after accounting for differences in race, sex, age, region of the country, season, income, and degree of urbanization. Changes in exposure to lead in paint or in the diet are unlikely explanations of the trend. However, the correlation of blood lead levels with the lead level in gasoline was highly significant (P less than 0.001) overall and in population subgroups defined by race, sex, and age. Although strong correlation does not prove cause and effect, the most likely explanation for the fall in blood lead levels is a reduction in the lead content of gasoline during this period.
Four hundred and sixty-six "blind" duplicate serum samples were analyzed for polybrominated biphenyls (PBB) by gas chromatography in two laboratories. As calculated by the regression model of Deming, the correlation coefficient for these 466 samples, whose values ranged from nondetectable amount to 1240 parts per billion was 0.9983. Prior to these analyses, the method was validated in both laboratories by using in vitro and in vivo PBB serum pools.
The transferability of the candidate Reference Method for total serum protein was tested in eight laboratories in the United States and Europe. National Bureau of Standards SRM 927 (bovine serum albumin) was used in each analytical run as the calibration standard. The mean absorptivity value obtained for this material was 0.2983 L g-1 cm-1. Four serum pools prepared at the Centers for Disease Control were analyzed on each of 15 days. Within-run variation of the protein values (expressed as CV) in the eight laboratories ranged from 0.1 to 2.5% and day-to-day (total) variation in six of the laboratories ranged from 0.4 to 1%.
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