Can hemoglobin concentration define anemia in women and men?

Provided that blood hemoglobin concentration (Hb_CONC) defines anemia, a relationship is expected between Hb_CONC and aerobic capacity (VO_2peak). This study tested the hypothesis that plasma volume intrinsically confounds the relationship between Hb_CONC and VO_2peak in the general population. Healthy women and men (n=461) across adulthood (18-78 yr) (41±18 vs. 42±18 yr, P=0.348) were included. Following 15 min rest in supine position, Hb_CONC was measured in antecubital venous blood. Total circulating Hb_MASS and plasma volume were determined via the carbon monoxide-rebreathing method. Peak pulmonary O₂ uptake (VO_2peak) was determined via incremental exercise. Hb_CONC was additionally measured immediately after the exercise test (Hb_CONCPEAK) in the majority of study subjects (n=417) to assess the influence of hemoconcentration. Linear regression analyses determined the relationship of hemoglobin variables with VO_2peak. Hb_CONC ranged from 10.1 to 15.0 g·dL^-1 in women and from 11.7 to 17.8 g·dL^-1 in men. Hb_CONC did not associate with VO_2peak in women (P=0.093) and men (P=0.165). Hb_CONCPEAK was positively associated with VO_2peak in women (r=0.15, P=0.038) and men (r=0.20, P=0.004). Hb_CONC adjusted by plasma volume positively associated with VO_2peak in women (r=0.23, P<0.038) and men (r=0.39, P<0.001). Hb_MASS positively associated with VO_2peak in women (r=0.34, P<0.001) and men (r=0.53, P<0.001). In conclusion, Hb_CONC is not associated with VO_2peak in women and men due to the intrinsic confounding effect of plasma volume. Hb_MASS, which is independent of plasma volume, associates with VO_2peak in both sexes and can thus sensibly define anemia in the general population.