Monday, August 15, 2011

Rubella Virus (RV) Laboratory Diagnosis

Nanotechnology and Advances in Medicine• Viral isolation
Culture of Rubella is technically complex and not routinely available (Mace et al., 2004). Viral isolation may occasionally be warranted, particularly to confirm infection during pregnancy or in neonates. In acute infection, the virus is isolated from nasopharyngeal secretions for about 6 days before and after the appearance of rash. Although virus can be isolated from circulating lymphocytes and affected skin, these sites do not yield virus frequently enough for clinical diagnostic purposes. Cord blood or placental tissue may be used to confirm congenital infection at time of birth. In addition, virus can be isolated from feces, and urine of the neonate. Later, the throat and CSF become the best sources of viral isolation because the infants with congenital rubella syndrome (CRS) shed fewer viruses (Best et al., 2005).
• Serology
Diagnosis of postnatal rubella and congenital infection (following birth) is normally carried out serologically by detection of rubella-specific IgM by ELISA. After postnatal rubella, IgM is usually detectable from the time of presentation until 5 to 6 weeks after the symptoms subside. Infants infected in utero produce rubella-specific IgM for many months after birth. Although tests for rubella-specific IgM are highly sensitive, they may be compromised by cross-reactivity with other viruses especially parvovirus B19 and Epstein-Barr virus (Weber, 1997).
Measurement of low-avidity rubella-specific IgG has been proposed as an alternative test for recent infection (Gutierrez et al., 1999). This test is based on the finding that the avidity of antibodies for their antigenic target increases with time. By inclusion of a mild detergent, such as diethylamine in the wash buffer of a standard IgG ELISA test, the avidity of the IgG can be measured (Bottiger and Jensen, 1997).
Other diagnostic serologic tests include measurement of HI or neutralization antibodies, but these are difficult to perform and are now used infrequently. ELISA tests, however, do not distinguish antibodies that are of functional significance, and patients may possess detectable ELISA antibodies in the absence of neutralizing or HI antibodies. In addition, analysis of the antigen specificity of ELISA-positive sera on immunoblots has shown that some patients have a skewed response to the major viral antigens E1 and E2. This is particularly notable in CRS patients but has also been shown in patients undergoing reimmunization (Meitsch et al., 1997).
• Molecular detection
Nucleic acid amplification techniques have also been developed for detection of RV- RNA in clinical samples. Sequencing of the amplified product provides confirmation that it is rubella specific (Katow, 1998). In the case of suspected congenital infection, in utero diagnosis using RT-PCR has been applied to samples obtained by amniocentesis, cordocentesis, or chorionic villus sampling. The technique is sensitive and specific in this setting (Costello and Yungbluth, 2007).

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