Blood and separated serum are the most common specimens taken to investigate the aetiology of communicable diseases. Venous blood can be used for isolation and identification of pathogens using sub-culture and inoculation techniques. Blood is also separated into serum for the detection of genetic material (e.g. using the polymerase chain reaction), specific antibodies, antigens, or toxins (e.g. by ELISA). For the processing of most specimens for diagnosis of viral pathogens, serum is preferable to un-separated blood except where otherwise directed. When specific antibodies are being assayed, it is often useful to collect paired sera, i.e. an acute sample taken at the onset of illness and a convalescent sample collected one to four weeks later. Blood can also be collected by finger prick for the preparation of slides for microscopy or for absorption onto special filter paper discs for analysis. Whenever possible, blood specimens for culture should be taken before antibiotics are administered to the patient.
NOTE: Collect acute and convalescent blood for serology between 2 and 4 weeks between acute and convalescent specimens.
NOTE: In some instances, it may be acceptable to use a special blood tube containing a separation gel, which encourages separation of serum from clot. In this case, the centrifugation step is eliminated. This has the advantage of ease and safety of specimen processing under field conditions. However, it is important to check with the laboratory in advance to ensure that these devices are appropriate for your particular investigation.
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