Staphylococcus Aureus Infection Workup

Updated: Jan 15, 2019
  • Author: Elizabeth P Baorto, MD, MPH; Chief Editor: Russell W Steele, MD  more...
  • Print
Workup

Laboratory Studies

An erythromycin-induction test, or D-test, should always be performed with staphylococcal sensitivities to reveal inducible clindamycin resistance among community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA).

Folliculitis, furuncle, and carbuncle

Make the diagnosis based on clinical appearance and, occasionally, on results of aspiration or incision and culture of purulent material from the lesion.

Osteomyelitis

Blood culture results are positive in only 30-50% of pediatric patients. Therefore, cultures of bone aspirate are useful in obtaining the organism and planning for long-term therapy. In addition, C-reactive protein levels and erythrocyte sedimentation rate are generally elevated in acute disease.

Septic arthritis

Examination of joint fluid, when obtained, is the primary means of diagnosis; the fluid should be sent to the laboratory for Gram stain and culture. In addition, the number and type of leukocytes should be determined. Median cell count in bacterial arthritis is 60.5 X 109 cells with a neutrophil predominance of greater than 75%. Often, synovial fluid glucose levels are low. Yield of culture may be improved by directly inoculating synovial fluid into blood culture bottles.

Endocarditis

The most important diagnostic procedure is the blood culture. Blood should be injected into hypertonic media if the patient has been exposed to antibiotics. Obtaining 3-5 sets of large-volume blood cultures within the first 24 hours is recommended.

Pneumonia

Blood culture findings for S aureus are more likely to be positive in secondary disease than in primary disease (90% vs 20%). Because blood culture results are often negative, an adequate respiratory tract specimen should be obtained prior to initiating therapy; specimens may include endotracheal sampling, pleural fluid, or lung tap. Sputum is not considered adequate because the organism is frequently present in the upper respiratory secretions of healthy individuals.

Thrombophlebitis

Although treatment is occasionally controversial, obtaining a blood culture through the intravenous line and a peripheral blood culture is usually recommended.

Next:

Imaging Studies

Ultrasonography may have value in identifying drainable skin and soft tissue foci of infection. [115, 116] Examples of radiographic findings in S aureus infections are shown in the images below.

Posteroanterior chest radiograph of a 15-year-old Posteroanterior chest radiograph of a 15-year-old with staphylococcal endocarditis and multiple septic emboli, revealing borderline cardiomegaly, multiple nodular infiltrates, and bilateral pleural effusions.
Lateral chest radiograph of a 15-year-old with sta Lateral chest radiograph of a 15-year-old with staphylococcal endocarditis and multiple septic emboli, revealing borderline cardiomegaly, multiple nodular infiltrates, and bilateral pleural effusions.

Examples of CT scan findings in S aureus infections are shown in the images below.

CT scan of the thorax (mediastinal windows) of a 1 CT scan of the thorax (mediastinal windows) of a 15-year-old with staphylococcal endocarditis and multiple septic emboli, revealing bilateral pleural effusions.
CT scan of the thorax (lung windows) of a 15-year- CT scan of the thorax (lung windows) of a 15-year-old with staphylococcal endocarditis and multiple septic emboli, demonstrating multiple nodular infiltrates (some with cavitation) consistent with septic emboli, volume loss on the right, and pleural effusions.
CT scan of the thorax (lung windows) of a 15-year- CT scan of the thorax (lung windows) of a 15-year-old with staphylococcal endocarditis and multiple septic emboli, revealing multiple nodular infiltrates (some with cavitation) consistent with septic emboli, volume loss on the right, and pleural effusions.
CT scan of the thorax (lung windows) of a 15-year- CT scan of the thorax (lung windows) of a 15-year-old with staphylococcal endocarditis and multiple septic emboli, revealing multiple nodular infiltrates (some with cavitation) consistent with septic emboli, volume loss on the right, and pleural effusions.
CT scan of the thorax (lung windows) of a 15-year- CT scan of the thorax (lung windows) of a 15-year-old with staphylococcal endocarditis and multiple septic emboli, revealing multiple nodular infiltrates (some with cavitation) consistent with septic emboli, volume loss on the right, and pleural effusions.
Follow-up CT scan of the thorax (lung windows) of Follow-up CT scan of the thorax (lung windows) of a 15-year-old with staphylococcal endocarditis and multiple septic emboli, revealing multiple nodular infiltrates (some with cavitation) consistent with septic emboli, volume loss on the right, and pleural effusions.
Follow-up CT scan of the thorax (lung windows) of Follow-up CT scan of the thorax (lung windows) of a 15-year-old with staphylococcal endocarditis and multiple septic emboli, revealing multiple nodular infiltrates (some with cavitation) consistent with septic emboli and evolution of the lesions over time.
Follow-up CT scan of the thorax (lung windows) of Follow-up CT scan of the thorax (lung windows) of a 15-year-old with staphylococcal endocarditis and multiple septic emboli, revealing multiple nodular infiltrates (some with cavitation) consistent with septic emboli and evolution of the lesions over time.

Osteomyelitis

On plain film radiographs, destructive bone changes are usually observed 2 weeks after infection. This is because a 30-50% reduction in bone calcium content is required before an osteolytic lesion is visible. The clinical diagnosis of osteomyelitis is most often supported by findings on bone scan with technetium Tc99m–labeled diphosphonate. Increased tracer uptake reflects the inflammatory process in the bone lesion. However, this modality is not as useful in neonates or after trauma or surgery. MRI is the best imaging modality for defining purulent collections and for planning surgery.

Septic arthritis

Plain radiographs show capsular swelling. They are most useful in revealing other causes of hip pain, such as Legg-Calve-Perthes disease. Radiographs should be obtained with the child in the frog leg position as well as with the legs extended and slightly internally rotated. Displacement of gluteal fat lines because of the swelling of the joint capsule is an early radiologic sign of septic arthritis. If a bone scan is performed, increased uptake on either side of the joint is visible. As pyogenic sacroiliitis is difficult to diagnose, the radiologic method of choice is MRI or CT scanning.

Endocarditis

Echocardiography is a valuable adjunct. Two-dimensional echocardiography is more sensitive than the M-mode technique, and it has been used to reveal vegetations in patients with negative culture results. However, because of variable sensitivity, a negative finding does not exclude endocarditis. Transesophageal echocardiography is more sensitive than transthoracic echocardiography in the detection of intracardiac vegetations. [117, 118, 119]

A multicenter study concluded that clinical risk stratification combined with a normal transthoracic echocardiogram (TTE) may be adequate to rule out infective endocarditis (IE) in most patients with Staphylococcus aureus bacteremia (SAB). [120, 121]

Pneumonia

No radiologic features are highly specific, but the chest radiograph may provide information, especially in demonstrating its progression. Radiographs of patients with primary staphylococcal pneumonia may reveal unilateral consolidation, while patients with secondary staphylococcal pneumonia are more likely to demonstrate bilateral infiltrates on radiographs. Early in the disease course, the chest radiograph may reveal minimal infiltrates, but, within hours, they rapidly progress. Pleural effusion, pneumatoceles, and pneumothorax are also common. In oncology patients, S aureus may cause pulmonary nodules. [6]

Previous
Next:

Procedures

Needle aspiration for culture may have utility. [122]

Previous
Next:

Histologic Findings

For scalded skin syndrome (Ritter disease), histologic examination of a skin biopsy specimen is the most helpful, because demonstration of midepidermal separation at the zona granulosa is diagnostic of this entity and excludes erythema multiforme in which dermoepidermal cleavage occurs.

Previous