When possible, tissue samples should be collected for culture. Although more invasive than fine-needle aspirates or swabs, tissue samples can have a higher sensitivity and specificity, particularly with deeper infections in humans.1,2
Fine-needle aspiration can be performed on deeper sites and are preferred over superficial swabs of draining tracts. The small volume collected and small area of the site sampled can limit sensitivity. Ideally, ≥2 samples should be collected, as positive results from >1 sample provide more convincing information of the clinical relevance of an isolated bacterial species.
Swabs are typically easier to collect and can be useful in many clinical situations. When performed properly, swabs are more prone to isolation of contaminants but yield fairly similar results as compared with other methods.3 Although tissue biopsy should be performed when possible, the following discussion focuses on collection of swabs, as this is the most common approach. Important concepts for wound sampling are presented in Important Considerations when Collecting a Specimen from a Wound.
Flocked swabs should be used when available, as they recover bacteria from infected sites as well as cotton- or rayon-tipped swabs but are more effective at releasing recovered bacteria into the culture medium.4
A properly collected swab often appears relatively clean, with only some blood-tinged fluid present (Figure).
Of note, fungal culture can be considered in any case, but fungal infections are uncommon. Fungal culture is more important in infections that do not respond as expected to antimicrobials, in infections with unusual clinical presentation or progression, or when cytology suggests fungal involvement.