Ouroboros discusses complications inherent in measuring telomere length: "regarded as a potential biomarker of aging, there is a growing body of evidence indicating that shorter telomeres are associated with various diseases, including cancer, infectious disease, psychological stress, and cardiovascular disease. In cardiovascular disease in particular this had led to the hypothesis that premature, or accelerated, aging of the vasculature is a major contributing factor. While a handful of papers have estimated telomere length in specific vascular tissues, the numbers and sizes of samples in these studies are usually small, due to the limitations in obtaining the tissues and cells of interest. Thus, telomere length in genomic DNA obtained from circulating leucocytes is routinely used as a proxy for telomere length in other tissues. However, the inflammatory processes involved in disorders such as cardiovascular disease can result in increased white cell turnover, raising the possibility that the shorter telomeres measured simply reflect recent replicative activity." Nothing in biochemistry is ever as simple as we'd like it to be.

Link: http://ouroboros.wordpress.com/2008/09/19/support-for-leucocyte-telomere-length-as-a-proxy-for-telomere-length-in-vascular-tissues/