infectious diseases
Culture
Specimen collection
Specimen collection is important. For diagnosis of infectious disease, the rule of thumb is sample where the infection is. For skin lesions, the leading edge, not the center, should be sampled. Use of swabs is discouraged. However, if a swab is used, a flocked swab is preferred because it can recover more specimen. Swabs used for Nucleic Acid–Based Identification Methods for Infectious Disease must be compatible for the specific molecular assay for which they are intended. The wrong type of swab can produce false-negative results. Wooden-shafted swabs are toxic to some viruses. Cotton-tipped swabs are toxic to some bacteria, including chlamydiae. Blood culturesrequire decontamination and disinfection of the skin (eg, povidone iodine swab, allowed to dry, removed with 70% alcohol). Multiple samples, each from a different site are generally used; they are taken nearly simultaneously with fever spikes if possible. Normal flora of skin that grows in only a single blood sample is usually interpreted as contamination. If a blood specimen is obtained from a central line, a peripheral blood specimen should also be obtained to help differentiate systemic bacteremia from catheter infection. Cultures from infected catheters generally turn positive more quickly and contain more organisms than simultaneously drawn peripheral blood cultures. Some fungi, particularly molds (eg,Aspergillusspecies), usually cannot be cultured from blood. The specimen must be transported rapidly, in the correct medium, and in conditions that limit growth of any potentially contaminating normal flora. For accurate quantification of the pathogen, additional pathogen growth must be prevented; specimens should be transported to the laboratory immediately or, if transport is delayed, refrigerated (in most cases). 