The non-iron-deficiency-related difference in hemoglobin concentration distribution between blacks and whites and between men and women.

The relation of hemoglobin concentration to transferrin saturation had two distinct phases in the data of adult men and women (aged 18-44 years) collected in the first National Health and Nutrition Examination Survey (NHANES I): 1) a stable, linear phase of hemoglobin concentration in the range of normal transferrin saturation, and 2) a dramatic fall in hemoglobin concentration as the degree of transferrin saturation decreased below the linear range. This study confirmed that over the linear range of the hemoglobin concentration-transferrin saturation curve, where subjects were adequately iron-nourished, whites had systematically higher hemoglobin concentration values than blacks (0.61 g/dl); also, males had higher values than females (1.895 g/dl). The effects of race and sex on hemoglobin concentration were additive. The variance of the hemoglobin distribution in blacks was also greater than that in whites. Socioeconomic status and smoking status could not explain the above race and sex differences. The impact on the prevalence estimate of reducing the hemoglobin cut-off by 0.5 g/dl is substantial enough to justify a separate standard for blacks.