Get ready for USMLE Step 2 CK with a question focusing on necrotizing soft tissue infections (NSTI). Can you describe and interpret the diagnostic workup for an adult presenting with an NSTI and identify the key diagnostic features that differentiate between NSTI, possible NSTI, and non-NSTI? Enhance your clinical skills with this critical assessment.
A 58-year-old woman presents to the emergency department for evaluation of 2 days of worsening left leg pain, fevers, and fatigue. The patient has a past medical history of IV drug use and reports that she recently injected heroin into her left foot. Temperature is 38°C (100.4°F), blood pressure is 132/60 mmHg, pulse is 104/min, respiratory rate is 18/min, and oxygen saturation is 97% on room air. The patient appears uncomfortable. Examination of the left lower extremity reveals erythema over the dorsal foot, induration, and moderate tenderness to palpation. No crepitus is palpated. The patient is started on IV fluids and broad-spectrum antibiotics. Laboratory findings are shown below. CT imaging demonstrates evidence of increased attenuation and stranding of the subcutaneous fat, as well as edema and fluid tracking along the superficial and deep fascia of the calf. Which of the following is the best next step in management?
| Laboratory value | Result |
| Serum chemistry | |
| Sodium | 129 mEq/L |
| Potassium | 4.5 mEq/L |
| Chloride | 97 mEq/L |
| Creatinine | 1.8 mg/dL |
| HCO3 | 18 mEq/L |
| Glucose | 343 mg/dL |
| BUN | 25 |
| CBC | |
| Hemoglobin | 11.5 g/dL |
| Leukocyte count | 18,100 /mm3 |
| Platelet count | 225,000/mm3 |
| Lactic acid | 3.6 |
| CRP | 25 mg/dL |
A. Emergent surgical consultation for debridement
B. Admission to the hospital for continued IV antibiotics
C. Discharge on cephalexin
D. Infectious disease consultation
E. Ultrasonography of the left lower extremity
Scroll down for the correct answer!
The correct answer to today’s USMLE® Step 2 CK Question is…
A. Emergent surgical consultation for debridement
Before we get to the Main Explanation, let’s see why the answer wasn’t B, C, D, or E. Skip to the bottom if you want to see the correct answer right away!
Incorrect answer explanations
Today’s incorrect answers are…
B. Admission to the hospital for continued IV antibiotics
Incorrect: This patient has CT evidence consistent with NSTI and a high LRINEC score and can be diagnosed clinically with an NSTI. The next step in management is emergent surgical consultation for debridement.
C. Discharge on cephalexin
Incorrect: This patient has CT evidence consistent with NSTI and a high LRINEC score and can be diagnosed clinically with an NSTI. The next step in management is emergent surgical consultation for debridement.
D. Infectious disease consultation
Incorrect: This patient has CT evidence consistent with NSTI and a high LRINEC score and can be diagnosed clinically with an NSTI. The next step in management is emergent surgical consultation for debridement.
E. Ultrasonography of the left lower extremity
Incorrect: This patient has CT evidence consistent with NSTI and a high LRINEC score and can be diagnosed clinically with an NSTI. The next step in management is emergent surgical consultation for debridement. An ultrasound is not necessary.
Main Explanation
This patient with a history of injection drug use presents for evaluation of fever, pain, erythema, and induration of the left foot. This patient’s clinical examination findings are concerning for a necrotizing soft tissue infection; however, no crepitus is palpated, and the patient has moderate and not severe pain, which makes the diagnosis in question. The next step in evaluation is to use the CT findings and LRINEC score to identify the likelihood of NSTI. If, as seen in this patient, the LRINEC is high (>8) or the CT shows findings consistent with NSTI (edema and inflammatory changes in the subcutaneous tissue), then the diagnosis of NSTI can be made clinically, and an emergent surgical consult should be called.
Initial bloodwork for patients with suspected NSTI should include cbc, cmp, lactic acid, blood cultures, inflammatory markers (e.g. crp/esr), and CK levels. Patients without evidence of crepitus and without rapidly progressing disease on physical examination should have blood cultures drawn, IV antibiotics administered, imaging performed, and the LRINEC should be calculated. Imaging results and the LRINEC score can be used to diagnose NSTI, possible NSTI, or non-necrotizing infections. Patients with crepitus or rapidly progressing physical exam findings require immediate surgical consultation for operative debridement.
Major Takeaway
Patients with a suspected NSTI but without crepitus or rapidly progressive findings on physical exam should have CT imaging performed and a LRINEC score calculated to assess the likelihood of NSTI and guide management.
References
- Bonne SL, Kadri SS. Evaluation and Management of Necrotizing Soft Tissue Infections. Infect Dis Clin North Am 2017; 31:497.
- Hakkarainen TW, Kopari NM, Pham TN, Evans HL. Necrotizing soft tissue infections: review and current concepts in treatment, systems of care, and outcomes. Curr Probl Surg. 2014 Aug. 51 (8):344-62.
- Hua C, Urbina T, Bosc R, et al. Necrotising soft-tissue infections. Lancet Infect Dis 2023; 23:e81.
- May AK, Stafford RE, Bulger EM, et al. Treatment of complicated skin and soft tissue infections. Surg Infect 2009;10:467–499.
- Misiakos EP, Bagias G, Patapis P, Sotiropoulos D, Kanavidis P, Machairas A. Current concepts in the management of necrotizing fasciitis. Front Surg. 2014. 1:36.
- Stevens DL, Bryant AE. Necrotizing Soft-Tissue Infections. N Engl J Med 2017; 377:2253.
- Stevens DL, Bisno AL, Chambers HF, et al. Practice guidelines for the diagnosis and management of skin and soft tissue infections: 2014 update by the infectious diseases society of America. Clin Infect Dis 2014; 59:147.
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