Keys To The Diagnostic Workup For Patients With Diabetic Foot Infections

Valerie L. Schade, DPM, AACFAS, and Nathan S. Higa, DPM, AACFAS

   Perform the test by having the patient lie flat on his or her back, and elevate the affected extremity above the level of the heart for three to five minutes. Erythema secondary to inflammation or peripheral arterial disease will dissipate with elevation of the limb. Erythema secondary to infection will not. Then demarcate the extent of erythema with an indelible marker and your initials, date and time/place to allow for assessment of progression or regression after the initiation of antibiotic therapy.

What A Close Reading Of Laboratory Studies Can Reveal

Complete blood count (CBC) with differential. A CBC with differential will allow assessment of the white blood cell count (WBC), hematocrit, hemoglobin and platelet count. Patients with diabetes and a moderate or severe diabetic foot infection present with a normal WBC more than 50 percent of the time.5-8,11

   However, a WBC >10.0 x 103 µL should raise concern for a deep space abscess or necrotizing type infection. Studies have shown that elevated WBC following medical or surgical intervention is an indicator of poor outcomes, necessitating further surgical intervention and/or amputation.3,5,18,19 Hematocrit and hemoglobin levels are often low in patients with chronic osteomyelitis.3,19 Consider transfusion in patients in whom amputation is warranted as the total blood loss from a partial foot amputation can be several hundred cubic centimeters.

   Basic metabolic panel (BMP). The BMP allows for assessment of electrolyte abnormalities, renal function and the patient’s current glucose level. Hyperglycemia may be one of the only markers of moderate or severe infection in patients with diabetes. Electrolyte abnormalities, particularly hypo- or hyperkalemia, must be stable prior to any surgical intervention. Assessment of renal function is crucial in the consideration of certain antibiotic dosing.

   Erythrocyte sedimentation rate (ESR). The ESR is an indirect measure of acute phase reactants, which increase in inflammatory or infectious conditions. An increase in acute phase reactants alters the normal negative charge of an erythrocyte, affecting the rate it descends in a sample of anticoagulated blood.8 Perform the test using the Westergren method by placing a diluted and anticoagulated sample of venous blood in a 2.5 mm diameter, 250 mm graduated cylinder. Then record the rate of erythrocyte sedimentation in millimeters per hour.8 Normal levels are 0 to 15 mm/hr for men and 0 to 20 mm/hr for women with a standard deviation of 4 mm/hr.8

   With increasing age, adjust the levels of ESR by dividing the age by two for men and adding 10 to the age and dividing by two for women.8 Factors that can elevate the ESR are age, female gender, pregnancy, oral contraceptive use, malignancy, myocardial infarction, morbid obesity, trauma and heparin use.8,20 Researchers have reported an ESR ≥ 70 mm/hr to be the optimal cutoff for positive correlation with the presence of osteomyelitis with a reported sensitivity of 90 percent, specificity of 94 to 100 percent, a positive predictive value of 81 to 95 percent, and a negative predictive value of 66 to 89 percent.8,19,20

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