Mixture analysis of erythrocyte lithium-sodium countertransport and blood pressure

This study employs multivariate normal mixture analysis, a technique for identifying discrete subgroups within populations, to examine the relation of erythrocyte lithium-sodium (RBC Li+-Na+) countertransport and blood pressure in a group of 474 healthy adults. After adjusting for effects of age, gender, race, height, and weight, univariate mixture analysis of the distribution of mean arterial blood pressure (MAP) revealed the presence of only one group, whereas the distribution of RBC Li+-Na+ countertransport values was composed of a mixture of two groups (p less than 0.00005). When bivariate mixture analysis was applied to the combined distribution of MAP and RBC Li+-Na+ countertransport, two commingled subgroups were identified (p less than 0.00005). The smaller group (19%) had significantly higher values for both MAP (108.7 +/- 16.7 mm Hg, mean +/- SD) and RBC Li+-Na+ countertransport (0.455 +/- 0.147 mmol Li+/l cells.hr) than the larger (81%) group (MAP 93.3 +/- 12.2 mm Hg, RBC Li+-Na+ countertransport 0.247 +/- 0.080 mmol Li+/l cells.hr, p less than 0.0001 for both differences). The relation of MAP to RBC Li+-Na+ countertransport was distinctly different in these two subgroups. In the larger group, we found a weak positive (r = 0.21, p less than 0.0001) correlation for unadjusted values, which was not significant after adjustment. The smaller group, with higher levels of MAP and RBC Li+-Na+ countertransport, showed significant negative correlations for both unadjusted (r = -0.28, p less than 0.008) and adjusted (r = -0.41, p less than 0.0001) values.