Elaine Sunderlin
EBM 6/25/2010
“Probing to Bone in Infected Pedal Ulcers”
M. Lindsay Grayson, MD, et.al.
JAMA;Vol 273, No.9;March 1, 1995;721-723
Background:
- Pedal ulcers occur in up to 25% of diabetic patients
- Important to determine difference between soft tissue infections alone and
soft tissue infections with underlying osteomyelitis
- What’s the best way to diagnose osteomyelitis?
Hypothesis: The presence of bone, via gentle probing, in the depths of infected
pedal ulcers in patients with diabetes is indicative of osteomyelitis
Type of Study: Prospective cohort study of hospitalized diabetic patients receiving
antibiotic treatment for severe limb-threatening foot infection.
Methods: During a 2 year period beginning in December 1998 , 92 patients with 97
infections were enrolled in a randomized, double-blinded, single-center prospective
clinical trial to study the efficacy of Ampicillin/Sulbactam versus
Imipenem/Cilastatin in the treatment of limb-threatening foot infections in diabetic
patients. This data pool was used in this follow-up study. Patients in the original
study without pedal ulceration, with nonhealed recent surgical wounds, or with
pedal infection that had been debrided in a manner to likely expose the adjacent
bone were excluded.
- In patients with open ulcers, probing to detect bone was performed prior to
debridement
- When ulcers were covered by an eschar, probing was undertaken after
debridement that was limited to removal of overlying eschar
- Probing was done using a sterile, blunt, 14cm stainless steel eye probe held
like a pencil. One of the two authors assessed the ulcer at bedside
- Bone was considered palpable (positive probe test) when on gentle probing,
the evaluator detected a rock-hard, often gritty structure at the ulcer base
without the apparent presence of any intervening soft tissue.
- Diagnosis of osteomyelitis
o Histologic (92%): presence of inflammatory cells within the bone,
fibrosis of intertravecular soft tissue, and destruction or necrosis of
bone and reactive new bone formation
 + radiographic (35%)
 + clinical criteria (46%)
o Radiographic alone (2%): bone destruction on imaging studies
o Clinical criteria alone (4%): identification of purulent friable
nonviable bone by the surgeon performing debridement
o Radiographic and clinical (2%)
Results:
Osteomyelitis
present
33
17
50
Positive test
Negative test
Totals
Osteomyelitis
absent
4
22
26
Totals
37
39
76
Specificity =
_______number of true negatives_________
# of true negatives + # false positives
22 = 0.66 (66%)
26
Sensitivity =
_______number of true positives_________
# of true positives + # false negatives
33 = 0.85 (85%)
50
Positive predictive ____________True positives__________
Value (PPV)
True positives + false positives
33__ = 0.89 (89%)
33 + 4
Negative predictive ____________True negatives_________
Value (NPV)
True negatives + false negatives
__22__ = 0.56 (56%)
22 + 17
Positive Likelihood
Ratio
______Sensitivity_________
1 – specificity
___0.66__ = 4.4
1 – 0.85
Negative Likelihood
Ratio
___1 – sensitivity________
Specificity
_1 – 0.66 = 0.4
0.85
Is the study valid? I think so. It is a cohort study with a small number of patients.
As they were testing a physical exam, there is always examiner difference. They
tried to limit that with only 2 observers performing the exam. The histology
specimens were only examined by 1 pathologist who was blinded to the probe test
results.
Would I use this in my clinical practice? Unlikely. The positive predictive value is
not robust. In general, the current diagnostic tools for osteomyelitis are limited, at
best.