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Background: The ankle-brachial index (ABI) is a popular method for evaluating peripheral artery disease (PAD). However, it is unreliable in patients with diabetes mellitus (DM), particularly in cases of media arterial calcification (MAC), where falsely elevated or unreliable values may be produced. The toe-brachial index (TBI) is therefore recommended in such cases, but has its limitations. The tissue optical perfusion pressure (TOPP) method is another automated diagnostic protocol combining oscillometric ABI measurement (oABI) and photo-plethysmographic pulse-wave assessment using the pulse wave index (PWI). The study evaluated TOPP-derived parameters in diabetic patients with or without MAC, in comparison with established functional vascular examinations. Methods: PAD patients with DM presenting in our outpatient clinic were enrolled prospectively from January to August 2024. Patients with peripheral bypasses or deemed unsuitable for the TOPP method were excluded. All patients received an ABI, TBI and TOPP measurement. Results: A total of 107 patients with DM were included in the present study. 38 patients presented with MAC and 69 patients without. The majority were male. Most patients presented with claudication (20 Fontaine stage IIa, 30 stage IIb), 9 presented with rest pain (Fontaine stage III), and 31 with wounds (Fontaine stage IV). 17 patients were free of symptoms (Fontaine stage I). The two parameters of the TOPP method, oABI and PWI, both correlated with the TBI and ABI. In patients with MAC, the oABI did not correlate with any other measurement, but the PWI did weakly correlate with the TBI. MAC is an important factor in influencing measurement accuracy. Despite their limitations, the TBI showed a significant correlation to the clinical symptoms (correlation coefficient = -0.387, p < 0.001). Conclusions: In patients without MAC, oABI and PWI correlated with ABI and TBI. TBI was the most reliable parameter in those with MAC. PWI correlated with TBI, but the correlation was weak. TBI should not be replaced by PWI. PWI may provide complementary information in a diagnostic protocol. oABI did not correlate with clinical symptom severity in DM patients, independently of the presence of MAC, and is unsuitable as a stand-alone parameter. A combination of TBI and TOPP-derived parameters may help to assess the severity of peripheral artery disease in diabetic patients with MAC. Larger multicentre studies are required.