3.3 When and How to Culture a Wound

Culture is a decision, not a reflex

Skill 010312 asks when a culture is needed and how to choose the most appropriate method; skill 020106 is the hands-on overlap you will see again in the intervention chapters. The governing idea is simple: chronic wounds are contaminated or colonized. Surface organisms live in slough and biofilm. If you swab every yellow venous ulcer on admission, you will collect Staphylococcus, mixed gram-negatives, and whatever survived the last dressing, then feel pressured to "cover" the report. That pattern drives unnecessary antibiotics, C. difficile, resistance, and delayed attention to compression and edema—the actual reasons the ulcer stalled.

Obtain a culture when the clinical infection question is already on the table:

• Spreading infection: advancing cellulitis, lymphangitic streaking, new satellite breakdown around the wound, crepitus, or suspected deeper collection.
• Systemic signs: fever, rigors, unexplained tachycardia, hypotension, or acute delirium in a frail adult when the wound is a plausible source.
• Failure to heal despite appropriate etiology-based local care (compression on a venous ulcer with adequate arterial inflow, offloading on a plantar diabetic ulcer, pressure redistribution on a sacral injury) once you have given that care enough time to work and you have ruled out the obvious mechanical failures.
• Before targeted antibiotics when you have already decided the patient needs systemic therapy, or when empiric therapy is failing and you need a better map of organisms and resistances.

Do not culture because the wound is chronic, because discharge planning asked for "a bug," because the dressing was malodorous before cleansing, or because you want a number to put in a box. If you will not change antibiotics, isolation, or debridement based on the result, skip the swab.

Cleanse first, then choose the method that matches the question

Irrigation or gentle cleansing with a non-antimicrobial (or facility-approved) cleanser before sampling removes dressing residue and loose planktonic organisms. Culturing the film on an old foam tells you about the foam.

Levine technique is the preferred swab method for an open wound bed. After cleansing, select viable tissue—usually clean granulation, not eschar and not a lake of pus. Rotate the swab over about 1 cm² for several seconds (classically about five) while applying enough pressure to express fluid from the tissue. That expressed fluid is the point: you are sampling organisms in the tissue, not the picnic on the surface. Place the swab in transport medium promptly and label site, time, and whether the patient is already on antibiotics (which can suppress growth).

Wrong swabs that mislead:

• Swabbing a pool of pus without pressing into viable tissue.
• Rolling across necrotic slough or eschar (you will grow whatever is digesting dead protein).
• Culturing periwound intact skin or a dressing.
• A dry swipe that never expresses fluid.

Abscess aspirate. If the clinical problem is a fluctuant pocket, a needle aspirate (by a credentialed clinician, with aseptic technique) samples the closed space. A surface swab over an un-drained abscess misses anaerobes and the true collection.

Punch or tissue biopsy is the gold standard when you need quantitative culture, when osteomyelitis is in question and you need bone histopathology and culture, or when swab and clinic picture disagree. Tissue is obtained from the wound bed or bone by a clinician privileged for the procedure, not from a casual swipe. Quantitative thresholds taught in older literature (such as 10⁵ colony-forming units per gram) are interpreted with the surgeon and laboratory—modern practice still treats biopsy as more truthful than a slough swab, especially in diabetic foot infection and non-healing atypical ulcers.

Send aerobic studies routinely; add anaerobic, mycobacterial, or fungal tests when the history fits (deep necrosis, water exposure, chronic nodules, immunocompromise, or failure of ordinary bacteria to explain the course). Write the clinical story on the requisition. A lab that only reads "wound" will not look as hard as one that reads "diabetic foot, probe to bone, preoperative bone biopsy."

Colonization versus infection, NERDS versus STONEES

Contamination is non-replicating organisms on the surface. Colonization is replicating organisms without host injury. Local (covert) infection is host injury limited to the wound. Spreading (overt) infection moves into surrounding tissue. Systemic infection involves the whole patient. Biofilm sits across this continuum and helps explain why a wound looks shiny, stalling, and resistant to a single antibiotic course without looking classically purulent.

Wound nursing commonly uses two bedside frameworks (Sibbald and colleagues) to decide whether you are looking at local bioburden or spreading infection. They are clinical tools, not a culture report.

Count clues rather than waiting for every letter. Three or more NERDS features support local infection; three or more STONEES features support spreading infection. Fever, leukocytosis, and hemodynamic change override any mnemonic: that patient is systemically ill.

Read the report with the limb attached

A culture that grows coagulase-negative staphylococci, mixed skin flora, or a light growth of Corynebacterium from a swab of slough is usually colonization. A heavy growth of S. aureus or beta-hemolytic streptococci from a Levine sample of viable tissue in a patient with spreading erythema is a different story. Pseudomonas on a moist venous ulcer may be a colonizer or a contributor to heavy exudate; treat it as a pathogen when the wound is deteriorating, the granulation is abnormally discolored, or the patient is ill—not because the organism is famous.

Treat the patient, not the printout. If NERDS features resolve with debridement and moisture control, you do not owe the lab an antibiotic for every named rod. If STONEES features plus fever are present, you do not wait 72 hours for speciation to start empiric therapy—but you still obtain the best specimen you can before or as you start antibiotics, unless delay would be dangerous.

When the report and the patient disagree, believe the patient and upgrade the specimen. A swab that "shows no growth" in someone on antibiotics with probe-to-bone disease is not a clearance of osteomyelitis. That situation needs bone biopsy and imaging, not a second slough swab.

Scenario: two swabs, one useful

A 64-year-old woman with a recurrent medial-gaiter venous ulcer is admitted from home. The covering nurse swabs a thick yellow layer without cleansing because "it looks infected." The lab returns mixed gram-negative flora and Candida, and a hospitalist starts a broad oral antibiotic plus fluconazole. ABI is 1.02, pulses are present, there is no fever, and the only new finding is leakage under a wrap that has not been applied in three weeks. A CWCN instead cleanses, documents no spreading warmth, no size increase, no probe to bone, scores NERDS (non-healing, exudate, slough, smell on the old dressing that vanished after irrigation), restarts compression, and holds systemic antifungals. Two weeks later the area is down 30% and no antibiotic was required.

Contrast a second patient: the same gaiter location, but now enlarging ulcer, 3 °C warmer than the other leg, new satellite breakdown, probe to bone, and fever. That is STONEES plus systemic signs. After cleanse, a Levine swab of viable tissue (or, better, operative bone if surgery is imminent) is indicated, empiric antibiotics start, and MRI is requested. Culturing the necrotic roof would have been the wrong specimen even though culture itself was the right idea.

Remember the hierarchy you will reuse in later chapters: clinical infection first, cleanse, Levine or aspirate or biopsy, then interpret the organisms as guests or invaders. The exam rewards that sequence over a reflex swab of every chronic wound.