Preservatives, such as boric acid for urine or polyvinyl alcohol (PVA) and buffered formalin for stool for ova and parasite (O&P) examination, are designed to maintain the appropriate colony counts (for urines) or the integrity of trophozoites and cysts (for O&Ps), respectively. Other transport, or holding, media maintain the viability of microorganisms present in a specimen without supporting the growth of any of the organisms. This maintains the organisms in a state of suspended animation so that no organism overgrows another or dies out. Stuart's medium and Amie's medium are two common holding media. Sometimes charcoal is added to these media to absorb fatty acids present in the specimen that could kill fastidious (fragile) organisms such as Neisseria gonorrhoeae or Bordetella pertussis.
Anticoagulants are used to prevent clotting of specimens such as blood, bone marrow, and synovial fluid because microorganisms will otherwise be bound up in the dot. The type and concentration of anticoagulant is very important because many organisms are inhibited by some of these chemicals. Sodium polyanethol sulfonate (SPS) at a concentration of 0.025% (w/v) is usually used, because Neisseria spp. and some anaerobic bacteria are particularly sensitive to higher concentrations. Because the ratio of specimen to SPS is so important, it is necessary to have both large (adult-size) and small (pediatric-size) tubes
available, so organisms in small amounts of bone marrow or synovial fluid are not overwhelmed by the concentration of SPS. Heparin is also a commonly used anticoagulant, especially for viral cultures, although it may inhibit growth of gram-positive bacteria and yeast. Citrate, EDTA (ethylenediaminetetraaceticadd), or other anticoagulants should not be used for microbiology, because their efficacy has not been demonstrated for a majority of organisms. It is the microbiologist's job to make sure the appropriate anticoagulant is used for each procedure. One generally should not specify a color ("yellow-top") tube for collection without specifying the anticoagulant (SPS), because at least one popular brand of collection tube (Vacutainer, Becton, Dickinson and Company) has a yellow-top tube with either SPS or ACD (trisodium dtrate/dtric add/dextrose); ACD is not appropriate for use in microbiology.
If specimens cannot be processed as soon as they are received, they must be stored (see Table 5-1). Several storage methods are used (i.e., refrigerator temperature [4°C], ambient [room] temperature 122° C], body temperature [37° C], and freezer temperature [either -20° or -70° C]), depending on the type of transport media (if applicable) and the etiologic (infectious) agents sought. Specimens suspected of containing anaerobic bacteria, for example, should never be stored in the refrigerator, while cerebral spinal fluid (CSF) should always be kept at 37° C. Urine, stool, viral spedmens, sputa, swabs, and foreign devices such as catheters should be stored at 4° C. Serum for serologic studies may be frozen for up to 1 week at -20° C, and tissues or specimens for long-term storage should be frozen at -70° C.
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