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Direct
Virus isolation Virus can grow on certain cell lines and they can produce cytopathic effects. These cytopathic effects can be observed under light microscope. Most viruses that grow on cell lines can be identified with the help of a unique type cytopathic effect. For example, the cytopathic effect of adenovirus is different from that of Herpes simplex virus. Virus isolation technique has generally a good sensitivity and specificity but requires a good quality specimen as viable virus particle are required to replicate. Some viruses replicate slowly and they may require 2-3 weeks of incubation to show the presence or absence of virus particles. The technique is cheaper but requires trained manpower. The advantage of the technique is that the isolated viruses can be further characterized for genotypes or resistance to drugs.
Specimens
Immunofluorescent Fluorescent labeled antibodies are used to detect intracellular virus particle or components. It is a rapid test and results can be available within 2 hours. It has a good sensitivity and specificity.
Specimens Respiratory symptoms- respiratory secretions Vesicular rash- vesicular swab
Antigen detection Latex agglutination assays It is sensitive and rapid. Cheaper and simple.
Specimen Stool for detection of Rotavirus, Norovirus
Enzyme linked Immuosorbent Assay It is sensitive and rapid It can be automated. Commercial tests are available and it is easy to make standardization.
Specimens Stool for detection of Norovirus and adenovirus Blood for HBsAg
Electron microscope It is rapid. It is not as sensitive as EIA or PCR. It is cheap but the capital cost is very high. Skilled manpower is required to carry out the test. It is a catch-all technique (more than one viruses can be detected)
Specimens Gastroenteritis - stool Urine for BK virus.
Molecular testing This is a method utilize the fundamental property of how the double strand are formed during replication and transcription that is the binding of each base to its complementary base. An oligonuceotide probes that bind to complementary genetic sequence are used to identify the organism. In this method amplification of the genetic sequence before detection may or may not be needed.
Polymerase chain reaction (PCR) This is a technique involve amplification of a genetic sequence in a sample. Two primers are used to amplify certain genomic region. Basically this method is highly specific and sensitive test. Potentially the technique can amplify a single genome, but in practice there are some hurdle and greater than 10 copies of the genome is required to have an valid result. Depending on the quality of the primer used the specificity can approach to 100 %. The technique also can be used to quantify the viral load. Viral load has been used to monitor response to antiviral therapy in patients infected with HIV and CMV. The disadvantage is that it is more expensive compared to the other tests. And yet more laboratories are introducing the test as it can reduce hospital stay as the test can be carried out in a few hours.
Specimens Any fluid or tissues can be processed. For blood borne viruses blood samples are appropriate. Send blood in EDTA Plasma- HIV viral load, HBV, HCV Whole blood- for viruses that are found in WBC, for example, CMV, HHV 6 and EBV. Respiratory secretions should be examined for respiratory viruses.
Polyacramide gel electrophoresis (PAGE) Extracted genomic materials are run on polyacramide gel, and depending on the size of the segments they travel different distance. The test is cheaper, however it is labor intensive and tedious.
Specimen Stool for Rotavirus.
Genotyping Automated sequencing method can be used to determine genotype and sensitivity to antiviral drugs. Certain mutations are associated with resistance to specific antiviral drugs. The test is generally expensive.
Specimen The specimen required for genotypic test is like that of the PCR.
Indirect
Antibody detection
IgG It appears in the first few days of infections and may remain detectable for several years. It is used to identify patients who have previous exposure e.g. immunity to chickenpox It can also be used to diagnose infection e.g. Hepatitis C antibody Seroconversion may be used to identify patients with recent infection.
IgM It appears in the first few days of infections and may be detectable only for 3 to 5 months. Indicates recent/ acute infection. False positive is not uncommon.
Serology Clotted blood Ř Latex agglutination CMV IgG, VZV IgG
Ř EIA HB core antibody HIV antibody
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Date this page is updated: 04/03/2007 23:13:30 www. virologynotebook.co.uk
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