Validity and Repeatability Characteristics of a Non-Invasive, Infrared-Based Method Estimating Plasma Indocyanine Green Decay in Healthy Dogs

SIMPLE SUMMARY: Dogs are affected by various types and degrees of liver disease and dysfunction. Indocyanine green is an indicator dye cleared from the bloodstream exclusively by the liver. Hence the rate of its disappearance can potentially indicate liver dysfunction in dogs. We compared a novel, non-invasive, infrared-based method of detecting indocyanine green with a reference test that requires repeated blood samples. We completed both methods in parallel in both conscious and sedated dogs. Furthermore, we repeated the noninvasive test in conscious dogs to assess its repeatability. There did not appear to be a clear association between the two methods, possibly owing to signal quality issues with the non-invasive method. In addition, the repeatability of the noninvasive method was poor. We conclude that the noninvasive method performed poorly in healthy dogs, which should be considered when investigating dogs with clinical liver disease. ABSTRACT: Plasma clearance of indocyanine green (ICG-CL) is an invasive method to evaluate liver dysfunction. We aimed to investigate the practicality of a noninvasive, transcutaneous, infrared-based method estimating the disappearance rate of indocyanine green (ICG-PDR). In a randomized, cross-over study, both ICG-CL and ICG-PDR were determined in eight healthy dogs while conscious and when sedated with medetomidine and medetomidine–vatinoxan. ICG-PDR was further repeated in six of the dogs to assess its repeatability. Differences were tested with repeated-measures analysis of variance and post hoc t-tests with Bonferroni corrections, while associations were evaluated by both Spearman and Pearson correlation analyses. Furthermore, repeatability was assessed by examining calculated coefficients of variation (CV). A significant decrease in ICG-CL was observed in dogs sedated with medetomidine, while no difference between conscious and sedated states was detected with ICG-PDR. Overall, correlations between ICG-CL and ICG-PDR were poor, as was the intrasubject repeatability of ICG-PDR in conscious dogs with CV consistently above 20%. While some of the results may be explained by poor signal quality for the non-invasive method, we conclude that in healthy dogs ICG-PDR performed poorly.