There are five approaches to the diagnosis of viral diseases by the use of clinical specimens: (1) identification of the virus in cell culture, (2) microscopic identification directly in the specimen, (3) serologic procedures to detect a rise in antibody titer or the presence of immunoglobulin (Ig) M antibody, (4) detection of viral antigens in blood or body fluids, and (5) detection of viral nucleic acids in blood or the patient’s cells.
The sensitivity and speed of several types of laboratory tests used to diagnose viral infections are described in Table 34–1.
TABLE 34–1Comparison of Sensitivity and Speed of Viral Lab Tests ||Download (.pdf) TABLE 34–1 Comparison of Sensitivity and Speed of Viral Lab Tests
| ||Viral Culture: Detection of Cytopathic Effect ||Polymerase Chain Reaction (PCR): Detection of Viral DNA or RNA ||Direct Fluorescent Antibody (DFA): Detection of Viral Antigens in Cells ||Rapid Antigen Test: Detection of Influenza Proteins |
|Sensitivity ||High (3+) ||High (3+) ||Intermediate (2+) ||Least (1+) |
|Relative speed when results become available ||Slowest ||Fast ||Fast ||Fastest |
IDENTIFICATION IN CELL CULTURE
The growth of viruses requires cell cultures because viruses replicate only in living cells, not on cell-free media the way most bacteria can. Because many viruses are inactivated at room temperature, it is important to inoculate the specimen into the cell culture as soon as possible; brief transport or storage at 4°C is acceptable.
Virus growth in cell culture frequently produces a characteristic cytopathic effect (CPE) that can provide a presumptive identification. CPE is a change in the appearance of the virus-infected cells. This change can be in such features as size, shape, and the fusion of cells to form multinucleated giant cells (syncytia). CPE is usually a manifestation of virus-infected cells that are dying or dead. The time taken for the CPE to appear and the type of cell in which the virus produces the CPE are important clues in the presumptive identification.
If the virus does not produce a CPE, its presence can be detected by several other techniques:
Hemadsorption (i.e., attachment of erythrocytes to the surface of virus-infected cells). This technique is limited to viruses with a hemagglutinin protein on their envelope, such as mumps, parainfluenza, and influenza viruses.
Interference with the formation of a CPE by a second virus. For example, rubella virus, which does not cause a CPE, can be detected by interference with the formation of a CPE by certain enteroviruses, such as echovirus or Coxsackie virus.
A decrease in acid production by infected, dying cells. This can be detected visually by a color change in the phenol red (a pH indicator) in the culture medium. The indicator remains red (alkaline) in the presence of virus-infected cells but turns yellow in the presence of metabolizing normal cells as a result ...