This paper assess the relationship between the quantity of three anti-seizure medications in hair and the color of the analyzed hair, while controlling for the effects of dose, dose duration, and patient age for 140 clinical patients undergoing anti-seizure therapy. Three drugs are assessed: carbamazepine (40 patients), valproic acid (40 patients), and phenytoin (60 patients). The relationship between hair assay results, hair color, dose, dose duration, and age is modeled using an analysis of covariance. The covariance model posits the hair assay results as the dependent variable, the hair color as the qualitative categorical independent variable, and dose, dose duration, and age as covariates. The null hypothesis assessed is that there is a no relationship between hair color and the quantity of analyte determined by hair assay such that darker colored hair will demonstrate higher concentrations of analyte than lighter colored hair.

The analysis reveals that there is a significant relationship between dose and concentration for all hair color categories independent of the other covariates or the categorical independent variable.

There does not appear to be any relationship between carbamazepine concentration and hair color. There is a weak relationship between hair color and valproic acid concentration, which the data suggest may be mediated by age. There is a significant, moderate relationship between phenytoin concentration and hair color such that darker colored hair has greater concentration values than lighter colored hair.