Modelling blood pressure as a continuous outcome variable in a co-twin control study.

The co-twin control study is a popular research design that is used in many research disciplines, including epidemiology, medicine, and the behavioural sciences. In co-twin control studies of monozygotic (MZ) exposure discordant twin pairs, the appropriate statistical tests used to analyse data from continuous outcome variables are the parametric paired sample t test or the non-parametric Wilcoxon signed rank test. Results from these tests enable researchers to discover if an independent variable (the exposure) is significantly associated with the dependent variable (continuous outcome measure) while controlling for genotype, age, and sex. To date, little work has been done on modelling continuous outcome variables in co-twin control studies. The general linear mixed model (GLMM) can be used to analyse several types of data, including repeated measurements, longitudinal, spatial, multivariate, and clustered data. As co-twin control studies yield clustered data (that is, each twin pair comprises a cluster and the members of the cluster are correlated), the GLMM provides a useful statistical approach for the regression analysis of data from co-twin control studies with continuous outcome measures. The GLMM incorporates fixed and/or random eVects parameters, and assumes that a continuous outcome variable is linearly related to a set of predictor variables while allowing for possible correlation between observations (that is, lack of independent observations) and the modelling of more than one error term. We investigated the applicability of a linear fixed eVects model in a co-twin control study of the eVect of tobacco smoking on systemic arterial blood pressure in a population-based sample of Swedish twins 80 years old and older. Methods This co-twin control study involved a secondary data analysis from the first measurement occasion of the OCTO-Twin Study, a five occasion, two year interval longitudinal study of Swedish octogenarian and nonagenarian twins that began in 1991. The OCTO-Twin Study has been described in detail by McClearn and colleagues. Smoking discordance for twin pairs was defined in a never smoked, ever smoked dichotomy (that is, one twin has never smoked, and the other is a current or former cigarette, pipe, or cigar smoker). Of the 351 twin pairs from the first measurement occasion, there were 11 male and 13 female MZ smoking discordant pairs. Systolic and diastolic blood pressure were measured by nurses using a mercury sphygmomanometer with subjects in a supine position after five minutes of rest. Paired sample t tests were conducted to discover if a history of tobacco smoking was significantly associated with the blood pressure measurements in smoking discordant twin pairs. SAS PROC MIXED was used to perform the regression analysis of the co-twin control data. The overall objective of the regression analysis was to quantify the eVect of tobacco smoking on a persons’s systolic and diastolic blood pressure while accounting for other potential explanatory factors. These factors included age, body mass index, a dichotomous variable indicating the current use/non-use of prescription antihypertensive medication, and a monthly alcohol consumption score. For the two zygosity by sex groups, separate variancecovariance matrices were modelled for the blood pressure variables. As it was not necessary to model fixed and random eVects simultaneously in our data analyses, random