19 Diagnosis of bacterial infections The identification and differentiation of bacteria principally relies:-on microbial morphology and growth variables on culturing the organisms on non-selective and selective media. Culture remains the standard for diagnosis of bacterial infection because it is the simplest,the most reliable and the most cost efficient method. Another advantage is the fact that parallel testing of the sensitivity towards antibiotics is possible.
20 n molecular-biological technics that provide rapid identification of infectious agents,both from culture isolates and directly within clinical specimens.- on the detection of the antibody response to the pathogen. The concentration of infection-sepcific antibodies is determined in titers or activity units.
21 In principle,the pathogen should be identified by culture in any acute infection that requires treatment.This,however,is only possible if the pathogen is still detectable in excretions,secretions,blood or tissue at the time when the clinical symptoms occur,e.g. salmonellae in feces in the case of diarrhea or gonococci from endocervical canal in genital tract infection.
24 Diagnosis of viral infections The primary diagnostic technique for most viral infections is the isolation of the virus in cell culture. Serologic techniques may also be useful, especially if the virus was isolated from a nonsterile site. In some instances, serologic diagnoses is the only practical approach in a clinical laboratory. Direct detection of antigen in body fluids or tissues has also been effective for some viruses. The list of agents for which direct detection of antigen is useful will undoubtedly continue to expand.
25 The compare of the advantages and disadvantages of each of these approaches: Culture:Time:Days to weeksAdvantages:Specificity and sensitivity maximum;isolate available for characterizationDisadvantages:Cell culture facilities needed;time for diagnosis may be long
26 Direct detection:Time:Hours to 1 dayAdvantages:Speed of diagnosis;used for viruses difficult to cultureDisadvantages:False-positives and –negatives;hard to batch testsSerology:Time:WeeksAdvantages:Assessment of immunity or response to virus isolated from nonsterile site; used for viruses difficult to cultureDisadvantages:Potential cross-reaction;need for acute and convalescent specimens
28 Direct ExaminationIt is highly recommended that a direct microscopic examination be made on most this provide an immediate presumptive diagnosis for the physician, but it may also aid in the selection of an appropriate culture medium.A phase-contrast microscope is a valuable adjunct in the direct examination of specimens. The advantages include the following:1)mounts can be made and examined quickly;2)there is no need for direct staining;and3)the objects can be clearly visualized.