System for Prediction of Recurrent DFU
approach also allows for unambiguous treatment of
participants who suffered multiple DFU events during
the study, whereas the more traditional approach of
aggregating data on a per-participant basis would
underreport the potential impact of the study device
for those patients at highest risk. Finally, using
2-month intervals for reporting better ensures causality
between the thermometric signal measured and the
development of any subsequent DFU given the long
duration of follow-up.
For the purposes of the ROC analysis, we considered
true-positive cases those in which a given temperature
asymmetry threshold was exceeded in any two
consecutive scans prior to the participant developing a
DFU in the same 2-month interval. False-positive cases
were those in which a given threshold was exceeded
but the participant did not develop a subsequent DFU
during the 2 months. Random 2-month intervals
of participant data were sampled to assess the false-
positive and true positive rates. We obtained each
2-month interval by randomly selecting a participant
and then randomly selecting a start date from which to
index a 2-month interval within period of participation.
Given the large number of potential 2-month intervals,
we conducted a sensitivity study to determine how
many samples were necessary to estimate the false-
positive and true-positive rates with confidence. We
increased the number of samples until the ROC
area under the curve converged to two significant
digits. Convergence was initially achieved at 10,000
samples. To justify pooling data across the seven sites,
we compared the observed DFU incidence (on both
a per-patient-year and a per-patient basis) among all
site-to-site permutations. We also compared site-bysite
incidence against the pooled estimate. None of the
comparisons yielded statistically significant differences
in incidence at a = 0.05. From this, we concluded any
center heterogeneity was negligible.
Figure 2: Comparison of thermometric data from a participant who did not develop a new DFU during
the study (left) with the data from a participant who did (right). In the thermograms, the plantar aspect
of the foot is viewed from below so that the right foot is at image left.
A
C
B
D
Ulcer
6
6
5
4
3
2
1
5
4
3
2
1
0
0
0
5
10
15
20
25
30
0
2
Weeks after Enrollment
Subject I
A
24 o C
B
27 o C
24 o C
4
6
8
10
Weeks after Enrollment
Subject II
C
27 o C
20 o C
D
23 o C
17 o C
20 o C
Current Pedorthics | September/October 2019
47