Pilot study on near-infrared spectroscopy in peripheral artery disease: Differentiating upper and lower limbs and its correlation with the ankle-brachial index

Peripheral artery disease (PAD) is a global health challenge, with current diagnostic methods, including the ankle-brachial index (ABI), having limitations, particularly in patients with arterial calcification. Near-infrared spectroscopy (NIRS) offers potential advantages as a non-invasive assessment tool, yet its clinical utility in PAD remains underexplored. This pilot study evaluated NIRS for differentiating between non-ischemic upper limbs and ischemic lower limbs, and assessed NIRS correlation with ABI. To do that, we performed an observational, cross-sectional study employing a convenience sample of 51 patients with PAD attending the vascular surgery outpatient clinic. A portable spectrometer recorded NIRS measurements from the right thumb and both halluces at rest. Random Forest classification was implemented to differentiate upper and lower limbs, revealing distinct NIRS patterns between upper and lower limbs, with an area under the ROC curve of 0.91 (95 % CI 0.88-0.94). Interval Partial Least Squares regression (iPLS) identified wavelength regions correlating with ABI, with the 1429-1463 nm interval being the most informative for ABI prediction, with a modest correlation (R2 = 0.167, RMSECV = 0.186). NIRS demonstrated strong discriminative capability between non-ischemic upper and ischemic lower limbs in PAD. While the correlation between NIRS and ABI was modest, it suggests potential clinical relevance. These findings indicate that NIRS could be a rapid, portable, non-invasive complementary tool for PAD assessment.