SPECIMEN SELECTION, COLLECTION, AND TRANSPORT

Results obtained from veterinary diagnosticians are in large part a direct reflection of the quality of specimen that was submitted. It is not always easy to obtain optimal specimens from animals, but application of a few basic principles should yield acceptable specimens that in turn yield high-quality microbiology results.

The first principle seems self-evident, but transgressions against it are common: specimens must be obtained aseptically from a site that is representative of the disease process. In some instances, the appropriate course is obvious; for example, urine culture is unlikely to yield clinically relevant information in a dog manifesting clinical signs of otitis externa. However, in some instances the choice of correct specimen maynot be quite so straightforward; a swab of nasal drainage might seem suitable for diagnosis of pneumonia, but in most cases a transtracheal wash is more appropriate (Table 2-1).

TABLE 2-1 Specimen Selection by Infection Site and Transport Devices

Swabs are the most commonly collected specimens, but they are generally not specimens of choice. They may become contaminated with commensal bacteria, and they provide a small sample volume when compared with aspirates, tissues, or body fluids. Bacteria may adsorb to fibers in swabs, further decreasing effective sample volume. Swabs can be useful for obtaining specimens from skin pustules, ears, conjunctivae, deep draining tracts or wounds, soft tissue infections, and the reproductive tract. It is advisable to avoid the wooden-shafted cotton-tipped variety because residual fatty acids and other potentially toxic substances in the fibers may inhibit recovery of bacteria, including chlamydiae. Plastic-shafted calcium alginate, rayon, or Dacron swabs are preferred.

Thorough microbiologic testing should include a direct microscopic examination as well as culture of the specimen, so it is important to collect a sufficient quantity of material to permit adequate examination. This requires, depending on the type of lesion, approximately 0.5 ml of aspirate or fluid, at least two swabs, 5 to 10 ml of blood, or about 1 cm³ of tissue.

Specimens must be collected at the proper time in the disease process, generally during the acute stage of infection and before initiation of antimicrobial therapy. Bacteria are often present throughout the course of the disease, such as in infections of the genitourinary tract. Other infections may be cyclical in nature and require planning to maximize recovery of the etiologic agent; for example, in animals suspected of having septicemia, blood cultures should be collected during a febrile spike.

Specimens should be placed in appropriate transport devices to maintain a buffered andnonnutritive environment to prevent metabolic damage to organisms of interest and overgrowth of contaminants (Table 2-2). It is imperative to keep swabs moist to prevent bacterial desiccation and loss of viability. If samples are to be cultured aerobically, swabs should be placed in Stuart’s, Cary-Blair, or Amies medium for transport.They should not be placed in sterile containers with bacteriostatic saline solutions or ethylenediaminetetraacetic acid (EDTA), which is bactericidal. Aspirated specimens may be left in the syringe (with the needle removed), or may be placed into sterile tubes. Tissue specimens may be transported in sterile tubes or in Whirl-Pak bags. Fecal specimens should be collected in sterile cups or bags, but if Campylobacter spp. is suspected, feces should be placed in Cary-Blair transport medium.

TABLE 2-2 Specimen Transport Systems

Use of a transport system is critical to avoid exposure to oxygen of specimens intended for anaerobic culture. Semisolid reducing medium is available in a variety of forms for the transport of fluids, swabs, and tissues, and is useful for recovery of aerobic, anaerobic, and microaerobic bacteria.

Refrigeration usually preserves viability and reduces overgrowth by extraneous organisms, but fastidious organisms (such as anaerobes)will sometimes die rapidly when exposed tolow temperatures. Specimens likely to contain these organisms, or samples of body fluidsother than urine, should be held at room temperature.

Legible labeling of specimens, with an indelible marker, is critical. Note the source and/orspecific body site, and provide relevant clinical information on the submittal form to facilitate appropriate specimen handling and interpretation of results. Information regarding the animal species and anatomic site of origin, the suspected causative agent, previous antimicrobial therapy, and clinical signs will help the laboratory make the best choices regarding media and incubation conditions. Delivery to the laboratory should be within 48 hours of collection. Regulations governing the shipment of clinical specimens are changing rapidly, and compliance with prevailing laws may be facilitated by consulting the U.S. Department of Transportation (www.dot.gov).