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Advanced Lipoprotein Testing in Young Adults
- Wendy S. Tzou, MD; and
- James H. Stein, MD
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IN RESPONSE:
Dr. Ziajka criticized the reproducibility of carotid IMT measurements in the Bogalusa Heart Study by comparing them with those in the ARYA study; however, this comparison is not valid. Our study's conclusions were based on a composite of common, bulb, and internal carotid IMT measurements, whereas the ARYA study reported common carotid IMT measurements only (1). Furthermore, the mean and standard deviation carotid IMT values in our study were higher than in ARYA; therefore, cross-study reproducibility comparisons are not reliable. Nevertheless, carotid IMT reproducibility in our study was similar to many recent studies, and measurement precision in the Bogalusa Heart Study has been strong enough to identify expected associations between carotid IMT and both traditional and emerging cardiovascular risk factors (2-4).
The major advance of the VAP-II test is better resolution of serum lipoprotein(a) and intermediate-density lipoprotein cholesterol levels (5). If this difference really was clinically important, improved prediction would have been expected in our study, but none of our models that incorporated lipoprotein(a), intermediate-density lipoprotein, or real LDL cholesterol levels (individually or in combination with each other or with LDL subclass) improved prediction of increased carotid IMT. Small differences between low and high carotid IMT categories are commonly observed in epidemiologic and clinical trials and identify important differences in cardiovascular risk. Small differences in carotid IMT are akin to small differences in blood pressure that are near the resolution of clinical measurements but predict major differences in cardiovascular outcomes.
Dr. Ziajka incorrectly stated that “More than two thirds of the study sample was healthy, very low-risk women.” Only 58% of our participants were women, and the cardiovascular risks in our study and in the ARYA study are not known. Event rates have not been reported for either study, and the young ages preclude accurate risk prediction using standard algorithms. This emphasizes the importance of using a well-validated surrogate marker, such as carotid IMT, for research in young adults. Directly measuring vascular changes that are associated with future cardiovascular events (instead of simply relying on blood tests with risk associations in older adults) allows atherogenesis in young adults to be studied in a meaningful way. It is well known that subclinical atherosclerosis begets clinical atherosclerosis and future cardiovascular events. Although there may be patient subsets in which advanced lipoprotein testing using vertical-spin density-gradient ultracentrifugation may be useful, it does not appear to be clinically necessary in most young adults.
Wendy S. Tzou, MD
James H. Stein, MD
University of Wisconsin Medical School
Madison, WI 53792
The Editors welcome submissions for possible publication in the Letters section. Authors of letters should:
•Include no more than 300 words of text, three authors, and five references
•Type with double-spacing
•Send three copies of the letter, an authors' form signed by all authors, and a cover letter describing any conflicts of interest related to the contents of the letter.
Letters commenting on an Annals article will be considered if they are received within 6 weeks of the time the article was published. Only some of the letters received can be published. Published letters are edited and may be shortened; tables and figures are included only selectively. Authors will be notified that the letter has been received. If the letter is selected for publication, the author will be notified about 3 weeks before the publication date. Unpublished letters cannot be returned.
Annals welcomes electronically submitted letters.
Article and Author Information
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Potential Financial Conflicts of Interest: Employment: J.H. Stein (Liposcience, Inc.).
