Repeatability of intravital capillaroscopic measurement of capillary density

The reliability of intravital capillaroscopy for determining capillary density (CD) of skin has been questioned because it depends upon the variability of the measuring process and subjective interpretation of data as well as the intrinsic heterogeneity of capillary spacing. The aim of this study was to assess the repeatability of a standardised method for measuring CD of the skin of the dorsum of foot. In each of 30 subjects (10 controls and 20 patients with peripheral vascular disease), the foot was systematically mapped by examining 20 sites on the dorsum of foot and 2 sites on each toe, using white light (native) videomicroscopy at 40 x magnification. Off-line analysis of videoprints was then undertaken to determine CD at each site, by counting capillaries within areas of acceptable photographic quality only, having first defined the criteria for counting capillaries. The mean values were then calculated and taken to represent the CD of the foot or toes. Repeatability of the measuring equipment was first assessed by noting the presence or absence of each corresponding capillary in 2 prints, taken at intervals of hours or days (in 10 subjects) or months (in 2 patients), of an identical area of skin which was marked by a microtattoo on the first occasion. On average, 95% of corresponding capillaries were identified in both prints (from controls and patients), thus implying little intrinsic temporal variation of capillary anatomy as well as excellent repeatability of the measuring equipment. Repeatability of data analysis was assessed by the same observer reading the same 20 prints in a blinded manner on three separate occasions (intraobserver repeatability), and 2 observers reading the same 24 prints (interobserver repeatability). The mean coefficient of intraobserver variation of CD estimate was 5.6% and the interobserver correlation coefficient was 0.94. Finally, overall repeatability of the method was assessed by repeating the procedure on a subsequent occasion (mean time interval of 5 days) in 10 subjects. The rate of agreement in mean CD between the two procedures [defined as 100- (difference between the two measurements/mean of the two measurements) x 100]% ranged from 86.4 to 97.1% (mean 93.5%). Thus using the above methodological technique, native capillaroscopy can be reliably used to determine CD of the dorsum of foot in comparing patient subgroups, as well as in longitudinal studies.