What are the ideal thresholds for ASTP, toe-brachial index in diabetic foot screening?

In diabetic foot screening (DFS), both absolute systolic toe pressure (ASTP) and toe-brachial index (TBI) can detect ankle-brachial pressure index (ABPI)-determined peripheral arterial disease (PAD), results of a Singapore study have shown. The optimal threshold values derived from a multiethnic Asian diabetic population are <0.7 for TBI and <95.5 mm Hg for ASTP.

“ASTP is an effective and efficient single measurement modality that may be used as an alternative to the well-established ABPI in DFS,” the researchers said.

This retrospective, observational study was conducted using DFS results from January 2017 to December 2017. Using the internationally accepted ABPI cutoff value of ≤0.9 to indicate PAD, the researchers conducted receiver operating characteristic analysis for ASTP and TBI.

In total, 1,454 patients (mean age 63.1 years, 50.8 percent men) were included in the study. Of these, 69.7 percent were Chinese, 13.5 percent Indian, 10.1 percent Malay, and 6.7 percent other ethnicities. [Ann Acad Med Singap 2022;51:143-148]

Areas under the curve (AUC) were 0.89 (95 percent confidence interval [CI], 0.85‒0.94) for ASTP and 0.94 (95 percent CI, 0.90‒0.98) for TBI, with the difference achieving statistical significance (p<0.001). Derived optimal threshold values to indicate ABPI≤0.9 were <95.5 mm Hg (specificity 0.86, sensitivity 0.84) for ASTP and <0.7 (specificity 0.89, sensitivity 0.95) for TBI.

“ASTP and TBI yielded AUCs in the excellent and outstanding categories of 0.8–0.9 and >0.9, respectively,” the researchers said. “These results supported the use of ASTP or TBI as alternative measurements to ABPI≤0.9 to indicate PAD for risk stratification in routine DFS in Singapore.”

The indicative threshold value of TBI at 0.7 for detecting an ABPI≤0.9 was consistent with internationally reported threshold values and guideline recommendations of TBI<0.7 to be indicative of PAD. [J Am Coll Cardiol 2017;69:1521]

Optimal threshold

“Our results provided further evidence to support TBI<0.7 as the optimal threshold in indicating ABPI-determined PAD in routine DFS in a multiethnic diabetic population such as Singapore,” the researchers said.

On the other hand, the derived threshold value of ASTP at 95.5 mm Hg for indicating ABPI≤0.9 is roughly 50-percent higher than the current recommended value of 60 mm Hg. [https://www.ace-hta.gov.sg/docs/default-source/acgs/foot-assessment-in-people-with-diabetes-mellitus-(june-2019).pdf]

“The current threshold of ASTP<60 mm Hg may be underdiagnosing many patients who may benefit from earlier detection and intervention during routine DFS,” the researchers said.

“As the results of this study are consistent with previously reported threshold values of 97 mm Hg to be indicative of PAD, the current recommendation of ASTP<60 mm Hg for indicating ABPI-determined PAD in DFS in Singapore needs to be reconsidered as more data and evidence become available,” they added. [J Foot Ankle Res 2017;10:58]

Of note, the difference in AUCs between ASTP and TBI favoured TBI, but ASTP could be considered for implementation in screening services due to its excellent AUC value and single parameter measurement characteristic, according to the researchers.

“ASTP is easier to perform and time-saving as brachial systolic blood pressure measurement is not needed when compared to TBI,” they said. “Time constraints have been reported to be a major deterrent to performing vascular assessments in practice.” [J Foot Ankle Res 2017;10:58; Vasc Med 2004;9:253-260]

In Singapore, the prevalence of diabetes is one of the highest in the world. Thus, there is a need for an accurate, time-saving, and efficient measurement to allow screening for ABPI-determined PAD during DFS that can also be carried out on a large scale. [Singapore Med J 2018;59:291-294]