Although not all infectious diseases require diagnostic laboratory testing, clinicians often rely on laboratory tests to help identify a causative agent, select an appropriate antimicrobial agent, and/or assess response to therapy. Children are more susceptible than adults to certain infections such as viral respiratory infections, streptococcal pharyngitis, and otitis media. The variety of potential pathogens and difficulties in collection of specimens create diagnostic challenges that are specific to children. Ordering the right test at the right time is paramount for laboratory diagnosis of infectious diseases and for preventing false-positive and false-negative results. Therefore, it is important that clinicians have a basic understanding of diagnostic microbiology so that they collect the optimal specimen and order the most appropriate test for diagnosing and selecting the most appropriate therapy for the infectious process.
COLLECTING AND PROCESSING BACTERIAL CULTURES
No test methodology or degree of laboratory expertise can correct the error of inappropriately collected and transported specimens. The proper collection and handling of clinical specimens is as important as selecting the correct medication for treatment. Common problems with clinical specimen collection and handling include insufficient quantity, contamination, inappropriate transport conditions, and delay in transport to the laboratory. Swabs, although commonly used, are less likely to yield reliable results, and their use for obtaining bacterial cultures should be discouraged, except when submitted for throat culture (or for rapid antigen tests, which include swabs as part of the test kit). Submission of sterile body fluid, scrapings, biopsy, or tissue samples increases the chance of recovery of pathogens and reduces the isolation of contaminants that may mislead or result in misdiagnosis. Table 221-1 provides important and useful tips for collection of specific specimens.
TABLE 221-1Key Recommendations From the Microbiology Laboratory |Favorite Table|Download (.pdf) TABLE 221-1 Key Recommendations From the Microbiology Laboratory
|Specimen or Culture Type or Test ||Recommendations |
|All specimens || |
Obtain all specimens for culture before initiating antimicrobial therapy.
Inform the lab if unusual organisms are possible (such as Brucella, Bartonella, etc).
|Cerebrospinal fluid (CSF) ||CSF samples submitted for bacterial culture should not be refrigerated. Samples submitted for molecular tests, preferred for detection of herpes simplex virus 1 and 2, enterovirus, and Mycobacterium tuberculosis, should be stored and transported at 4°C. |
|Blood culture ||Collect the appropriate volume per bottle. Volume of blood collected is the most important factor in successful isolation of the causative organism. |
|Synovial fluid ||Inoculate fluid into an aerobic blood culture bottle, preferably at the bedside, in addition to submitting fluid in a sterile container for Gram stain. |
|Abscess ||Submit tissue, fluid, or aspirate. Swab is not the specimen of choice. |
|Stool samples ||Ova and parasite exam are of low yield; do not order routinely. |
|Urine culture ||Urine should not sit at room temperature for more than 30 minutes. Hold at refrigerator temperature if delay is expected. |
|Skin and soft ...|