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Diagnostic Study - Description & Definition


A blood culture is a specific type of laboratory test whereby a sample of blood is extracted from a vein in the arm using a needle or fingerprick. The blood sample is exposed to growing media and incubated. Gram stains can be applied to the culture to more specifically identify the organism causing the infection.

Historical Overview

The first blood cultures were developed in the early 1900s.


Blood cultures and wound cultures are used to detect systemic infections. Culture results can confirm that an infection is present; it can also identify the micro-organism causing the infection. To obtain a blood culture, a sample of blood is injected into ≥1 vials containing media specific for aerobic and anaerobic organisms. The vials are incubated at body temperature for 5 days. If a vial is positive, a Gram stain is applied to identify the bacterium. The blood is also sub-cultured onto agar plates for 3 days to isolate the organism and identify the bacterial species. After the specific bacterium is identified, appropriately sensitive antibiotics are prescribed as treatment.  A wound culture is obtained by swabbing the purulent wound drainage.


Cellulitis of the hand

If there is a secondary abscess, a blood culture is recommended1; however, the results are only positive in 5-15% of patients. If there is no abscess, the abscess should under go a surgical incision and drainage. The abscess drainage should be cultured.  Once cultures have been obtained, infection should be treated empirically until laboratory culture and sensitivity reports allow more accurate antibiotic choices.

Paronychia of the finger

Paronychia of the finger frequently requires incision and drainage. The purulent drainage should be cultured to identify the specific organism causing the infection. Then the antibiotic sensitivity testing results can be used to identify a culture-specific antibiotics that can be used to treat the infection.

Felon finger

The literature contains little data regarding the management of felon finger. In one case report2, it was proposed that patients at low risk for MRSA should receive a first-generation cephalosporin. Patients at high risk for MRSA should receive vancomycin on an inpatient basis or 2 trimethoprim/sulfamethoxazole on an outpatient basis. When an examination reveals fluctuance or palpable fluid in the fingertip, an abscess is diagnosed. The abscess is incised and drained.  The pus should be sent for culture and sensitivity testing. In the meantime, the patient is treated empirically. X-rays should be done to rule out secondary osteomyelitis of the distal phalanx. If the patient’s symptoms fail to improve and the culture shows an infection that is not susceptible to the prescribed antibiotics, the antibiotic treatment should be changed and repeat debridement considered.  Frequently hospital admission for intravenous antibiotic treatment is done initially.

Normal Study Findings - Video
Diagnoses Where These Studies May Be Used In Work-Up (with abnormal findings images)
  1. 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:e10-52.
  2. Tannan SC, Deal DN. Diagnosis and management of the acute felon: evidence-based review. J Hand Surg Am 2012;37:2603-4. 
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