Plasma von Willebrand Factor Biological Activity
The biological activity of von Willebrand factor was determined by the ristocetin cofactor assay. Washed normal platelets (5 X 10′ cells) were subjected to agglutination in the presence of either normal plasma or plasma from pulmonary hypertensive patients (50 (jlL) and of threshold ristocetin concentrations ranging from 0.35 to 0.5 mg/mL. The agglutination response (ristocetin cofactor activity) was determined by the change in light transmission at 4 min. Results were expressed as a percentage of the response obtained with normal plasma in the same assay.
Analysis of Data
In general, results are expressed as the mean (± SD), although median values and ranges are also provided when appropriate. Comparisons of data obtained at the beginning of the study between patients and control subjects or between patient groups were made using the Student’s t test. Canadian health & care mall Click Here Differences between patient subgroups were tested by analysis of variance with appropriate multicomparison tests. To identify possible indicators of poor short-term prognosis, variables were subjected to univariate and multivariate analyses. Possible correlations of sex, age at the beginning of the study (< 14 vs > 14 years), and the etiology of pulmonary hypertension with patient status at 1 year were tested using the x2 statistics and Fisher’s exact test. Differences between survivors and nonsurvivors regarding age, mean pulmonary arterial pressure, and plasma vWF: Ag were tested using the Student’s t test. Variables that were identified as possible predictors of prognosis by these initial methods then were subjected to logistic regression analysis (S AS/ST AT® pack; SAS Institute Inc; Cary, NC). Selection was performed by a stepwise procedure, and estimated parameters were used for curve fitting. A significance level of 0.05 was assumed.