Department of Pathology, The University of Hong Kong, Queen Mary Hospital, Hong Kong

Diagnosis of viral infection in formalin fixed and paraffin embedded tissues is often problematic. In the past, many traditional diagnostic methods for viral detection are not applicable to histopathological specimens. With the advances in molecular biology techniques, new tests based on Polymerase Chain Reaction (PCR) and In Situ Hybridisation (ISH) are now frequently used to detect the presence of viral nucleic acids within fixed tissues. Despite the cross-linking fixation, DNA and RNA in tissues can still be detected by their hybridisation to specific probes. Although DNA in fixed tissues is often degraded to small fragments of less than one thousand base pairs, it can still be extracted, purified and amplified by specific PCR procedures. These technical advances have allowed the diagnosis of viral infection in fixed tissues. For examples, ISH and PCR are often used to detect Epstein Barr Virus (EBV) and Human Papillomavirus (HPV) respectively. The molecular tests for EBV and HPV have been applied in a number of clinical settings. For examples, the identification of EBV (and other viruses) in immuno-compromised or post-transplant patients, and the detection of EBV in human tumours known to be associated with the virus are often required. The needs to detect HPV in biopsy and cytology specimens are also common. For HPV, apart from detecting the presence of the virus, subtyping the HPV responsible for a particular infection may also have considerable clinical significance.