**3.4 Protein binding**

Protein binding may change with age and concomitant illness. In certain circumstances, an understanding of protein binding may be needed to interpret the data from a blood level measurement and to determine appropriate dose adjustments. In vitro plasma protein binding studies can determine the extent of binding of the parent and the major active metabolite(s) and identify specific binding proteins, such as albumin and alpha-1 acid glycoprotein. Optimal estimates of the degree to which protein binding is linear may be obtained by testing maximumand minimum observed concentrations (Buxton & Benet, 2011).

The main reason that age affects drug action is that drug elimination is less efficent in newborn babies and in old people, so that drugs commonly produce greater and more prolonged effects at the extremes of life. Other age-related factors, such as variation in pharmacodynamic sensitivity, are also important with some drugs.
