Wound Swab: Collection, the Levine Technique, and When Not to Swab

An open wound is always covered in bacteria. That single fact makes the wound swab one of the most misread specimens in the lab, because a swab dragged across the surface of any chronic ulcer will grow something, and that something is usually colonizing flora, not the cause of infection. The value of a wound swab depends almost entirely on two decisions made before the swab is sent: is this wound actually infected, and was it sampled in a way that reaches the infecting organisms rather than the surface colonizers.

This article is about open wounds, ulcers, surgical sites, burns, and diabetic foot ulcers. For a closed abscess or a collection you can aspirate, the aspirate is the better specimen and the pus sample article covers that. The rule is: if you can aspirate pus or take tissue, do that. Swab only when you cannot.

First decision: is the wound infected, or just colonized?

Every open wound is colonized. Colonization is bacteria living on the wound surface without causing harm. Infection is bacteria invading viable tissue and provoking a host response. A culture cannot tell you which one you are looking at; only the clinical picture can. So the first question is not what to swab, but whether to swab at all.

Sample a wound for culture only when it is clinically infected or is chronic and failing to heal. Signs that point to infection rather than colonization include increasing pain, spreading redness (erythema), warmth, swelling, purulent discharge, a foul smell, and a wound that is deteriorating or not healing as expected. Swabbing a clean, healing wound produces a list of colonizers that can trigger unnecessary antibiotics and drive resistance.

The core principle: a wound swab answers "which organisms are causing this infection," not "is this wound infected." The clinician decides infection; the swab only names the organism.

Second decision: sample the right way (the Levine technique)

If the wound is infected and no tissue or aspirate can be taken, the swab must be done well. The difference between a good and a useless wound swab is technique, and the evidence-based method is the Levine technique.

Before sampling:

• Debride the wound if appropriate, and clean the surface thoroughly with sterile saline (not an antiseptic, which would kill the organisms). Remove all superficial slough and exudate. This step separates surface colonizers from the infecting organisms underneath.

The Levine technique:

• Rotate the swab over a 1 cm² area of clean, viable tissue near the center of the wound.
• Press firmly enough to express fluid from the deep tissue, for about 5 seconds.
• The pressure is the point: it draws up deep exudate, so the swab samples the tissue, not just the surface.

Done this way, a Levine swab recovers organisms and colony counts close to what a tissue biopsy would show. Compare this with the older Z-technique (zig-zagging the swab across the whole wound surface), which samples more surface flora and performs worse.

What to avoid:

• Do not swab dry, necrotic tissue, slough, or pus crust. These carry colonizers, not the invading organisms.
• Do not roll the swab across the wound surface without cleaning first. That is the classic error that grows contaminants.
• Do not use an antiseptic to clean the wound just before swabbing; it suppresses the very organisms you want to grow. Use sterile saline.

When a swab is the wrong specimen: tissue biopsy and aspirate

The swab sits at the bottom of the specimen hierarchy for wounds. Better specimens exist and should be used when the wound and the situation allow.

Diabetic foot ulcers and deep or non-healing wounds deserve tissue, not a surface swab. A properly performed Levine swab is a reasonable alternative when biopsy is not possible, but a superficial swab of a diabetic foot ulcer is close to worthless and can send treatment in the wrong direction.

Sample the advancing margin or the base, not the middle of old slough. Wherever you sample, target viable infected tissue at the edge or depth of the lesion. In a burn, organisms are unevenly distributed, so sample several areas.

Anaerobes and the swab problem

Many wound infections, especially deep, chronic, foul-smelling, or bite wounds, involve anaerobes. Anaerobes survive poorly on a swab exposed to air. This is another reason tissue or aspirate beats a swab: a capped aspirate or a tissue sample in the right transport protects anaerobes far better. If anaerobes are suspected and only a swab is possible, use an anaerobic transport swab and get it to the lab fast.

Transport

• Deliver the swab to the lab as soon as possible, ideally within 30 minutes, at room temperature.
• If a delay is expected, place the swab in Amies transport medium so it does not dry out; a liquid-based swab (eSwab) is a good option because one collection can serve Gram stain and culture.
• Tissue for culture goes in a sterile container with a little sterile saline to keep it moist. Never put tissue for culture in formalin, which kills everything and makes culture impossible (formalin is for histopathology, not microbiology).
• Refrigeration is acceptable for routine wound swabs if delayed, but not when anaerobes or fastidious organisms are suspected; keep those at room temperature and transport quickly.

Processing Wound Swab in Microbiology Lab

Every open wound is colonized, so the laboratory's task is to judge whether what grows represents infection or just normal flora. The Gram stain and the pattern of growth are the tools it uses to do that.

Direct examination. The Gram stain is genuinely informative here and helps to differentiate colonization-verus-infection:

• The organisms present (their Gram reaction and morphology), which gives an early clue to the likely pathogen.
• The host response, meaning the number of pus cells (neutrophils). Abundant pus cells with a single predominant organism point toward infection, while scanty pus cells with many different organisms point toward colonization or contamination. The presence of squamous epithelial cells suggests surface or skin contamination rather than a deep sample.

Culture media. The swab is inoculated onto media that cover the usual wound pathogens:

• Blood agar and MacConkey agar for common aerobes and gram-negative bacilli.
• An anaerobic medium and an enrichment broth (such as thioglycollate) for deep, necrotic, chronic, or bite wounds, where anaerobes are common. As noted above, a swab recovers anaerobes poorly, so tissue or aspirate is preferred when anaerobes matter.
• Sabouraud agar where a fungal wound infection is suspected.

Incubation. Plates are incubated at 35 to 37 degrees C for 24 to 48 hours, with anaerobic cultures held longer.

Reading and interpretation. This is where the swab's limitations show, and where the interpretation connects back to the collection decision:

• Growth is assessed semiquantitatively (light, moderate, heavy) and against the Gram stain. A heavy, near-pure growth of a recognized pathogen with abundant pus cells supports infection.
• A light, mixed growth of several organisms, especially skin and environmental flora, usually reflects colonization or contamination rather than infection.
• Because a surface swab so often grows colonizers, the result is always read together with the clinical picture. A wound that is clinically infected but grows only mixed flora may have been sampled poorly (surface rather than depth).
• The laboratory should report the likely pathogen and its significance rather than an undifferentiated list of every organism recovered, since reporting colonizers can drive unnecessary antibiotics.

Identification and antimicrobial susceptibility testing

Following organisms are commonly isolated from wounds, and the first approach for their identification is:

• Staphylococcus aureus: gram-positive cocci in clusters, catalase and coagulase positive; the commonest wound pathogen, including methicillin-resistant strains. See the Staphylococcus aureus article.
• Streptococcus pyogenes (Group A) and other beta-hemolytic streptococci: gram-positive cocci in chains, beta-hemolytic; important in spreading soft-tissue infection. See the Streptococcus pyogenes article.
• Pseudomonas aeruginosa: oxidase-positive, non-lactose-fermenting gram-negative rod with a green pigment and characteristic smell; common in burns and chronic wounds. See the Pseudomonas aeruginosa article.
• Enterobacterales (Escherichia coli, Klebsiella, Proteus): gram-negative bacilli separated on MacConkey by lactose fermentation; Proteus swarms on blood agar. Read about Enterobacteriaceae in this article.
• Anaerobes (Bacteroides, Clostridium, anaerobic cocci): common in deep, necrotic, and bite wounds; grown on anaerobic media. Read more about anaerobic culture media in this article.

How to Remember

Every wound is colonized; the clinician decides infection. A swab names the organism, it does not diagnose infection. Swab only wounds that are clinically infected or chronic and non-healing.

Clean first, then press: the Levine rule. Debride and rinse with sterile saline, then rotate the swab over 1 cm² of viable tissue with firm pressure to express deep fluid. The pressure is what turns a surface smear into a real sample.

Tissue beats swab, aspirate beats swab. If you can take tissue or aspirate pus, do it, especially in diabetic foot and deep or chronic wounds.

Sample the edge or the base, not the slough. Viable infected tissue at the advancing margin holds the invading organisms. Dead slough and crust hold colonizers.

Saline to clean, never antiseptic. Antiseptic before swabbing kills the organisms you are trying to grow. Clean with sterile saline only.

Formalin is for the pathologist, not the microbiologist. Tissue for culture goes in saline. Formalin kills everything.

Key exam facts in one table

Where Students Get Confused

"The swab grew bacteria, so the wound is infected, right?" Not necessarily. Every open wound is colonized, so a swab almost always grows something. Whether that represents infection is a clinical judgment based on signs like spreading redness, pain, purulence, and failure to heal. The swab names the organism; it does not diagnose infection.

"A swab is a swab, so why does the technique matter so much?" Because a swab rolled across an uncleaned surface samples colonizers, while a Levine swab, taken after cleaning and with firm pressure over 1 cm² of viable tissue, expresses deep fluid and recovers the actual infecting organisms.

"Why clean with saline and not an antiseptic before swabbing?" Because antiseptic kills the organisms you are trying to identify. Cleaning removes surface colonizers so you can reach the infecting organisms underneath; sterile saline does this without suppressing the culture.

"For a diabetic foot ulcer, is a surface swab good enough?" No. Diabetic foot and other deep or chronic wounds deserve a tissue biopsy, because the infecting organisms are in the tissue. A properly performed Levine swab is an acceptable alternative when biopsy is not possible.

"The culture missed the anaerobes we expected, why?" Anaerobes survive poorly on a swab exposed to air. A tissue sample or a capped aspirate protects them far better. When anaerobes are likely (deep, chronic, foul-smelling, or bite wounds), avoid a plain swab if tissue or aspirate can be obtained.

"Can I send the tissue in the same formalin pot as the histology sample?" No. Formalin kills all organisms and makes culture impossible. Tissue for culture goes in a sterile container with a little saline; formalin is only for histopathology.

References and further reading

1. Tille PM. Bailey & Scott's Diagnostic Microbiology. 15th ed. St. Louis: Elsevier; 2022.
2. Leber AL, editor. Clinical Microbiology Procedures Handbook. 4th ed. Washington, DC: ASM Press; 2016. DOI: 10.1128/9781683670438.CMPH
3. Procop GW, Church DL, Hall GS, et al. Koneman's Color Atlas and Textbook of Diagnostic Microbiology. 7th ed. Philadelphia: Wolters Kluwer; 2017.