Interrater and intrarater reliability of photoplethysmography for measuring toe blood pressure and toe-brachial index in people with diabetes mellitus

BACKGROUND: A reliable tool to measure arterial flow to the feet in people with diabetes is required given that they are particularly prone to peripheral arterial disease. Traditionally, the ankle brachial index (ABI) has been used to measure arterial circulation, but its application is limited due to calcification of larger arteries. More recently, toe pressure and the toe brachial index (TBI) has been suggested as superior to ABI measurements because they assess smaller digital arteries less prone to arterial calcification. However, reliability studies for the clinical use of photoplethysmography (PPG) in people with diabetes are lacking. METHODS: Sixty people with diabetes mellitus (35 males and 25 females, mean age 59.6 yrs) consented to take part in the study. The majority (92%) had type 2 diabetes and 8% had type 1 diabetes. Forty-three percent were diagnosed as having peripheral neuropathy when tested using a biothesiometer and 15% were current smokers (10 – 40/day). A podiatrist and a diabetes educator measured toe and brachial blood pressure independently and in a random order using PPG. These measurements were repeated again seven days later, and subsequently analysed with intraclass correlation coefficients (ICC), 95% confidence intervals (CI) and standard error of measurement (SEM). RESULTS: The intrarater reliability of measuring toe pressures was excellent (ICC(3,1 =)0.78-0.79, SEM 8 mmHg) and interrater reliability was also excellent (ICC(2,2) = 0.93, SEM 4 mmHg). The intrarater reliability for measuring brachial pressures was generally poor (ICC(3,1) = 0.40 – 0.42, SEM 19 mmHg) and interrater reliability was fair-good (ICC(2,2). 0.65, SEM 14 mmHg). The TBI intrarater reliability was fair-good (ICC(3,1) = 0.51-0.72, SEM 0.08), whilst the interrater reliability of TBI was excellent (ICC(2,2) = 0.85, SEM 0.07). CONCLUSIONS: Based on these results, interrater and intrarater reliability of PPG is excellent for measuring toe blood pressure, good for TBI and only fair for brachial pressures in people with diabetes mellitus.