Identifying Foot Infection
Be vigilant in identifying foot infection
Foot infections are a common precursor to the decision for leg amputation. Infection can cause tissue necrosis and systemic illness. It is therefore critical to identify foot infection early.
Foot infection should be assumed present unless proven otherwise in all patients with any of the following findings:
| Category: | Finding: |
|---|---|
| symptoms | nausea |
| anorexia | |
| subjective chills or sweats | |
| new onset foot pain, especially in patients with baseline sensory neuropathy | |
| signs | altered mental status |
| fever | |
| tachycardia | |
| erythema | |
| fluctuance | |
| any drainage (not only purulent) | |
| blisters / bullae | |
| foot or calf edema | |
| supranormal laxity of a joint (suggests a joint capsule that isn’t intact | |
| foul odor | |
| laboratory results | leukocytosis: elevated white blood cell count |
| elevated neutrophil-to-lymphocyte (NLM) ratio | |
| elevated serum procalcitonin | |
| elevated c-reactive protein | |
| acute kidney injury | |
| hyponatremia | |
| bacteremia | |
| summative scoring system | criteria for of SIRS or septic shock |
| 2+ points on qSOFA | |
| imaging findings | cortical erosions |
| joint subluxation | |
| soft tissue gas |
Suspect infection if any of these symptoms/signs/findings are present. These findings are specific but not sensitive: in other words, absence of these findings does NOT rule out infection.
Here’s a brief (5 minute 8 second) video primer on identifying foot infections:
Accurately describing a foot infection in words
The instigating ulcer
Foot infections enter the soft tissue plans, joint spaces and bony structures of the foot via a foot ulcer (a.k.a. “wound”): a full-thickness epithelial defect located distal to the malleoli. The first step in describing an infection is therefore describing any ulcers on the foot:
- Location: use medial/lateral/plantar/dorsal, toes/forefoot/midfoot/heel, distal/proximal, and/or glabrous/non-glabrous. Also in relation to anatomic structures (ex. “second toe, over the dorsal aspect of the proximal interphalangeal joint).”
- Size: in fractions of inches, but centimeters if you must.
- Depth/base: expressed as a layer (dermis/fat/fascia/bone) or at least what you see at the base (tendon, debris, etc.) or overlying it (ex. eschar).
Local and regional signs of infection
Name any of the aforementioned exam signs you see, along with where they are in relation to the instigating ulcer. Most structures in the foot are oriented in a longitudinal direction, and infection often tracks longitudinally along these structures. Note that plantar ulcers sometimes track through the forefoot at the level of the distal metatarsals, sometimes causing fluctuance and drainage on the dorsal side of the foot.
Then describe any abnormal findings or pertinent negatives from the x-rays.
Systemic inflammatory and infection signs
The aforementioned systemic signs seen on exam, followed by abnormal lab results.
Tips to improve your ability to identify infection
Regarding the unmerited focus on pus
Please note that pus is not the sine qua non of infection.
First of all, not all infections are purulent. Consider gangrenous cholecystitis, for example. While MRSA and MSSA have been associated with fluctuance, other organisms – such as gram negative and anaerobic bacteria – more often cause a soft tissue necrosis response.
Second, pus is not the problem, but rather a reaction to the problem. Whereas Galen might have been proud to see you squeezing a foot and draining pus to release the excessive humor of phlegm, you – as a contemporary surgeon in-training – should know that surgical treatment should drain and treat the underlying infection.
Regarding the unmerited emphasis on the neutrophil count
Leukocytosis is has mediocre specificity and poor sensitivity. In our study of 184 episodes of moderate to severe foot infections, leukocytosis was present in only 27%. You should therefore NOT rely on leukocytosis to indicate the presence of a foot infection.
Regarding osteomyelitis
Patients do occasionally manifest systemic inflammatory signs due to osteomyelitis alone – i.e. without any accompanying soft tissue infection. A foot x-ray (ordered as “foot” or “toes”) is therefore NECESSARY to rule out infection.
Regarding gangrene and eschars
Seeing gangrenous tissue (including eschar) that is “dry” does not rule out infection.
Admission orders
Patients with systemic signs of infection or concerning local signs should be hospitalized.
Admission laboratory studies
- Type and SCREEN
- CBC with differential
- Complete metabolic panel
- C-reactive protein (CRP)
- procalcitonin
- brain naturetic peptide (BNP) if new dyspnea/orthopnea, if abnormal EKG or if history of heart failure
- PT/PTT/INR only if on anticoagulation
- Nares swab for MRSA
Admission medications
The acronym “SAVE THe LIMB” will help you remember medications that may be important for hospitalized with foot infections:
| Letter | Medication |
|---|---|
| S= | Statin medication |
| A= | Aspirin 81mg Antibiotic: ceftriaxone 1gm IV q24h ± topical cadexomer iodine to bedside for subsequent dressing changes |
| V= | Vitamin C 500mg PO BID and Vitamin D3 25mg PO daily Vancomycin with calculated dosage only if MRSA nares swab is positive OR if pre-op. for abscess I&D. |
| E= | ”Ensure” BID (though Glucerna SR has lowest added sugar) |
| T= | Tylenol PRN for pain or temperature >100.4F |
| He= | Heparin subcutaneous q8h. Do not hold for OR. Home meds except metformin, Coumadin, other anticoagulants. |
| L= | Lactobacillus 2 caps PO BID |
| I= | Insulin: 50% of long-acting insulin home dose + sliding scale PRN |
| M= | Multivitamin 1 cap daily |
| B= | Blood pressure meds, baseline + at least one PRN for SBP>140mmHg |
Empiric antibiotic treatment
Ceftriaxone is the primary empiric antibiotic given to patients who have not yet had operative treatment or whose culture results are pending.
Do NOT use vancomycin for the empiric treatment of methicillin-resistant Staphylococcus aureus (MRSA). It should instead be reserved for patients going to the operating room for incision and drainage of an abscess and/or those patients with MRSA found on nasal swab at admission. The prevalence of MRSA is 22% with either abscess or MRSA+ nasal swab, and 58% with both abscess and MRSA+ nasal swab. When there is no fluctuance/abscess and negative MRSA nasal swab testing, the prevalence of MRSA is only 5% of our patients.
Do NOT use pipercillin-tazobactam (Zosyn©) or cefipime for the empiric treatment of Pseudomonas, as this organism represents only 3% of isolates at our hospital. The combination of pipercillin-tazobactam with vancomycin poses risk for acute kidney injury.