Allometric scaling of flow‐mediated dilation: is it always helpful?

Summary Flow‐mediated dilation ( FMD ) is calculated as the greatest percent change in arterial diameter following an ischaemic challenge. This Traditional % FMD calculation is thought to have statistical bias towards baseline diameter (D base ), which is reduced by allometric scaling. This study examined whether allometric scaling FMD influenced the difference between a group of healthy young and older adults compared to the Traditional % FMD , and to determine whether a New (allometric) scaling % FMD improved the ability to obtain individually scaled FMD . Popliteal artery FMD was assessed in 18 young (26 ± 3 years) and 17 older adults (77 ± 5 years). ‘Corrected’ mean FMD was generated from a log‐linked ANCOVA model. Individual % FMD was evaluated using three calculations: (1) Traditional % FMD calculation; (2) Atkinson (allometric) scaling % FMD (peak diameter ); and (3) New scaling % FMD . Traditional % FMD was significantly larger in young (5·82 ± 2·58%) versus old (3·72 ± 1·26%). ‘Corrected’ FMD means (Y: 5·97 ± 2·12%; O: 3·98 ± 2·06%) were similar to Traditional % FMD ; however, the logarithmic transformation prevents statistical interpretation of group differences. Individually scaled % FMD using the Atkinson scaling resulted in values that were corrected for variations in D base but that were twofold to threefold larger than those of the Traditional calculation. New scaling % FMD resulted in values that were similar to values expected (Y: 6·21 ± 2·75%; O: 3·98 ± 1·36%); however, it did not effectively correct for variation in D base . Recommendations regarding the advantages of allometrically scaling % FMD should be made with caution until research clearly establishes the benefits of this approach.