  Capillary refill time (CRT) is a method for evaluating peripheral perfusion
by visual assessment. CRT is especially useful for quick evaluations in the
absence of sophisticated equipment. However, there are repeatability and
reproducibility limitations with CRT, especially for dark skin. To test the
limits of CRT repeatability and skin color independence, we developed a system
and method to perform simple and robust CRT measurements. The system consists
of an RGB camera and an LED lamp, with crossed circular polarizers imaging to
attenuate the light reflected by the superficial layer of the skin. The
capillary refill time is determined using an exponential regression on the
time-dependent green channel mean pixel intensity of the region of interest
after the compression is released. We limited this regression up to a
data-dependent cut-off time, after which we assume the exponential model is
invalid, and used the confidence interval of the uncertainty to develop a
criterion to flag and discard faulty measurements. We tested the system on
twenty-two volunteers with skin phototypes ranging from I to VI on the
Fitzpatrick scale, applying to their forearms a 7 kPa compression for 5 s.
After the release of measurements flagged as inadequate (about 20\% of
measurements) by our regressions, our results indicated good precision, with
high repeatability for all skin phototypes. Approximately 80\% of measurements
fall within $\pm 20\%$ of the individual's expected value for CRT (mean CRT
value). Our results suggest CRT can be used as a quantitative measurement and
encourages further developments for the implementation of a similar method on
smartphone cameras for quick and robust CRT measurements in patients' triage,
monitoring, and telehealth.
